I read a book (it was definitely a comic, possibly manga) about a girl whose parents die and she goes to live with her auntie and uncle. They are actually vampires (or witches). Her cousin is mean to her and the girl wants to leave but is convinced by her uncle to stay.
This sounds a little like one of the Courtney Crumrin stories by Ted Naifeh, maybe COURTNEY CRUMRIN AND THE NIGHT THINGS (the first mini-series in that series, published 2003)
Courtney goes to live with her uncle, who is a warlock/witch. She's initially not keen on the move, but eventually bonds closely with her uncle, and stays with him.
Here's the cover of the first issue:
There are differences from what you describe, but those may be explicable:
Courtney's parents are alive, and they come to live with the uncle, too. However, they virtually ignore her, and eventually are written out and leave her with the uncle. The bond she develops with her uncle is much more significant.
There's no cousin. Courtney is not accepted at school, so you could be thinking of one of her classmates.