-D-vil is a colony of the country that imposed the Separation (Tamsin is the up-and-coming leader with a growing Cult of Personality); D-vil sided with the Separation because they were intensely inbred and very close in terms of society (see utopian communism) - everyone knows everyone else, whether they enjoy each other or not (and of course, some know others better than others do), and most everyone is related to everyone else. D and T are special because they are one of the few pairs of full-siblings.
I.E., incest is the norm and the fact that T is marrying #1 and not D is unusual (frowned upon by some) but not heavily opposed. Recall that full siblings are not common to begin with, so it is technically not normal custom to marry someone quite THAT related to you. Also note that because of their uniqueness, T and D were brought up very closely.
In this society, marriage =/= sexual devotion. It determines who you live with, to whom you devote your time. But sex is an open subject and the making of babies fairly casual.
-Tamsin, as specified earlier, has this strange cult of personality. It's true that she is fiercely dedicated to the system (though she doesn't really have any imagination and thus isn’t going to be changing the world any time soon) and excessively passionate when it comes to her duties - and her brother.
Most people that know Tamsin aren't particularly fond of her (despite the raving cult of personality); Dalziel does, but isn't overtly amorous (at least, until some point later in Part 1). She and Matthew continually cross each other (most often concerning the Separation or - not coincidentally Tamsin's other passion - Dalziel).
Physically, Dalziel has never been in the best of health. Though this does not stop him from doing regular work, he comes down with some illness or another far more often than most. This was what initially led Tamsin to her obsession with him; if he so much as sneezed, he would be under her constant surveillance for the next day or two. She also forbade him from going out on any patrols too far from home - or from her (or Matthew; despite their general polarity and regular disagreements, she often entrusts him with providing companionship for Dalziel). This estrangement from the regular activities of boys his age (in addition to his general aversion to people to begin with) further isolated him. He has spent most of his life in the company of adults. (A good comparison of their relationship would be Touma and Yuki, or Aunt Queen and Tarquin, from their respective books. Obviously, T and D’s relationship does not strictly follow either example, but one can understand the general idea.)
Dalziel has nothing of his sister's pragmatism, nor her passion. He tends not to react to bad news or disaster as most would (and he is somewhat bothered by this detachment), but he does sympathize - and in T's eyes, OVERsympathizes - with those to whom he has no connection. (Morai-naki.) He is far from a dull character, however; he has a wild imagination (though he lacks the drive to put it to any use at the start of the story) and is open to radical theories (as he has no staunch beliefs).
Tamsin sees her brother as a break in reality - that someone cannot possibly be all right and be like this. And though she loves him to the point of coddling him, she cannot deny that she is eager to see him change. So obsessed with this, T often finds it necessary to prod D into doing things he wouldn't normally do, or point out things he has done differently of his own accord (though these are hardly profound - 'you wore the blue cloak instead of the red one'). Her interference with his life is not wholly negative - it DOES open him up to a wide array of different experiences.
It is revealed later in the story (prior to T's wedding but before the prologue [in a chronological sense; obviously the prologue comes first in the actual book]) that Matthew thinks that in Tamsin's desperate attempts to 'change' him, she is stifling him. That in her vulture-like observation and study of her brother, she is keeping him apart from the real world. When he questions her ability to accept D as he is, she replies that 'change is good,' to which he argues that 'CHANCE is good'.
Later on, after the wedding (not directly after, but a few days, weeks) Matthew attempts to reawaken T. At some point, he says 'change is good'; Tamsin loses it and punches him in the face. (Note: Try to find some less pathetic-sounding phrases for said ‘punch’…) In a later scene, it is revealed that he accidentally bit off a part of his tongue, which accounted for most of the blood. In the present scene, he's crying and whimpering and spitting out small shavings of tooth. And as pathetic and un-macho as that sounds, it’s a perfectly reasonable reaction – he is in pain. Tamsin is immediately contrite, though the maddening urge to pound something into the ground has yet to dissipate. This is the first of a series of random violent outbursts, and increased disorder in T's life.
MORE MARRIAGE NOTES
Matthew was opposed to T's marriage from the start, because he felt that it would change something that needn't be changed. He and D were the closest people to T, and he felt that if another was introduced, he would be somehow 'less close' to T. Matthew likes to be everyone's closest companion, everyone's confidant, and essentially the one who holds all the knowledge (though he has no desire for the potential power knowledge might give him). He also has no romantic interest in Tamsin. He simply doesn't want to be second in line when Tamsin has something to confess/ask/obsess over.
Matthew once asked Tamsin whether she valued all she was 'throwing away' (as she was preparing for her wedding). Becoming wed would forever seal away things like ruling their little civilization, or going off on patrol runs. [To add later: Why on earth would marriage be so constricting if sex remains a liberal form of recreation after marriage?] Everything T had worked for up until then would be for naught. And as the entire reason for marrying so late was because she had no intention of EVER marrying, because her work was her passion and it meant the world to her, why is she so eager to throw it away now?
Of course Tamsin is willing. She loves #1. (Ironically, they never marry, and her reputation and entire life are thrown to the sidelines anyway.)
And how does that work, exactly?
Three months before the weeding, #1 needs to go on a routine patrol run. He will be back and the wedding will take place immediately thereafter. T is suitably overjoyed, and spends the entire three months planning, re-planning, and fantasizing over their wedding. These three months are the happiest moments in Tamsin's story (and quite possibly the entire novel). This makes it all the more difficult when the procession comes back sans half the patrol - #1 included. Rather, he comes back in a box.
As one of the survivors (the captain) explains what happened - one of the cities near the forest mountains revolted, the first people to harness magic offensively, etc. etc. ambushed the patrol in hopes of eradicating them (motives at this point unknown) - Tamsin is going into varying stages of disbelief, terror, and anger. This is especially poignant because she was so very happy leading up to this instant.
With #1 dead, she refuses to cancel the wedding (the festivities of course are done away with, but the actual ceremony continues). She cuts her hair along with his, braiding it together. She sleeps with the braid for one night, and throws it in with the grave the following morning. In D-vil, inhabitants cut their hair at exactly two points in their lives - marriage and death. A braid is formed from the two marriage cuts and it remains in the couple's home until the death of a spouse. Then it is buried with the deceased and the marriage is considered null and void.
As one might expect, Tamsin is never quite the same, though after her period of intense sorrow (most of which is mercifully omitted from the text, as the narrative will focus more on Dalziel and Matthew), she does manage to be mostly optimistic, if slightly less methodic and calculating.
Multiple times in the novel, Matthew serves as the catalyst to her negative emotions. (thinking D a lost cause, thinking T is forcing D into something he isn't, disapproving of her marriage, trying to console her after #1's death, his frequent delves into either petty magic (which is considered useless and taboo in D-vil) or violations of the separation.
Matthew himself was not originally from D-vil. Instead, he and his father were refugees from some distant town. Apparently, they escaped from something awful that happened to M's mother. After said awful mishap (which M doesn't even remember), M's father became increasingly doting, and spoilt M rotten as if to atone for whatever happened to Mother. (This is partly why M thinks that T's treatment of D is detrimental, though he can hardly say he didn't turn out just fine.) As a result, M remains spoilt to this day, and finds it incredibly difficult to tune out desires or cope with not getting what he wants, when he wants it.
He harbors an intense affection for apples, and frequently violates the Separation with strings of blackmarket ships that bring exports from other cities (apples in particular). Though he has made an effort in the past to stop - just for Tamsin - he cannot. He wants, and he will go to any measure to receive.
Matthew is in direct contrast to T, not only in terms with his relationship with D or his general personality, but because Tamsin is the leader. Tamsin is the pragmatic, powerful one. The rule-abider. The special child (see above note about regular incest). Matthew is one of the Lost People (those caught in their non-native city at the time of the Separation - again, it is unspecified whether M's dad did this on purpose), and is thus shunned by the more elitist D-vil denizens. He is also known for getting into a lot of trouble (see Separation violations), and escapes consequences only because of his close relationship with Tamsin. He's also not a particularly good patrol officer, despite his skill one-on-one in battle. Basically, he is at the bottom of the political ladder, whereas Tamsin is climbing to the top.