I haven't read the novel but based on the spoilers i've read in the comments she is basically the twin's biological half sister because their mother are twins. But in terms of family relationship they are cousins???
so biologically, they are cousins since their moms are twins (is that what u mean?) but since she was adopted into the fam she is now technically their sibling
Im sorry but i was really glad reading it and it wasnt him!!! Call me crazy but i freaking love this!!!
Sammmeee