the book has become yellow from the sides and inside because of the walls in my house otherwise book in good condition.

This is how the books goes:

So. Were you glad, deep down? Were you glad to be rid of her? Your perfect sister? Were you secretly glad when she was killed? Following a horrific tragedy that leaves her once perfect family devastated, Katherine Patterson moves to a new city, starts at a new school, and looks forward to a new life of quiet anonymity. But when Katherine meets the gregarious and beautiful Alice Parrie her resolution to live a solitary life becomes difficult. Katherine is unable to resist the flattering attention that Alice pays her and is so charmed by Alice's contagious enthusiasm that the two girls soon become firm friends. Alice's joie de vivre is transformative; it helps Katherine forget her painful past and slowly, tentatively, Katherine allows herself to start enjoying life again. But being friends with Alice is complicated - and as Katherine gets to know her better she discovers that although Alice can be charming and generous she can also be selfish and egocentric.
Sometimes, even, Alice is cruel. And when Katherine starts to wonder if Alice is really the kind of person she wants as a friend, she discovers something else about Alice - she doesn't like being cast off. Shocking and utterly absorbing,