rosier family headcanons before I loose my train of thought:
old money to a point where they never have to mingle with society ever again; secluded and therefore out of touch with modern society and social etiquette; considered weird by others
they originated in France, but moved to an old estate in Northern England while the twins were still toddlers
classist with severely inbred structures; eventually the inbreeding got so bad that barely any children survived, which is why they allowed outsiders to marry into the family to stabilise their genetics
guardians and keepers of old and occult knowledge; incredibly superstitious, which makes their mannerisms not only stiff but eerie and uncanny
mr rosier (the twins' father) is one of these outsiders, who married into the family (the only reason the twins did not end up inbred); before the marriage he had to undergo several trials to prove his loyalty to their cause of knowledge keeping
their father is half-french, half-egyptian, making the twins mixed
had evan been a cis male instead of ftm, he and dora would have been married; since he is not (and his parents also ignore his existence altogether), mr and mrs rosier spent most of the twins' youth looking for a fitting suitor for dora
while the twins do love each other, it is never incestuous or more than a strong platonic bond as is usual between siblings (sorry, moots. i'll be sitting out on the rosecest)
opalite eyes that look colourless at first, but break the light like a prism into hundreds of different shades and glimmers, alongside the blonde to white-blonde hair, are considered the trademark of a "true" Rosier; given that Evan's eyes are the exact opposite of that, it played further into the alienation from his family
mr and mrs rosier spoiled dora to no end, given that she was their long-awaited heir; they have a hard time telling her no, which is why, despite their previous efforts, they never put a veto on dora's marriage to xeno (for one, he probably would have passed the trials anyway, and secondly, even if they put a veto on the marriage, dora's stubborness is much stronger than their will)
while evan harbours practically no feelings towards his parents, dora loves them, even though she, too, sees them as a pair of loons rather than truly authoritative parental figures
the twins both inherited their thirst for knowledge through their family's occupation, even if the scientific nature is much more prominent in dora than it is in evan; they are the first in their family to break the cycle of seclusion and abandon the cult-like traditions of their home, thus making them the generation of change
