Skin by Adrienne Maria Vrettos
Donnie sister has died. 1st page. She has starved herself literally to death. Then go back in time. Donnie and Karen spend a lot of time huddled on the back porch, summer and winter. They listen to their parents fight, it is so common, they keep a lunch box full of sweets always handy. Until Karen makes a friend out of the new neighbor, Amanda. Then it kind of just becomes Donnie. He tries to keep it together as everything falls apart. Trying to make friends at school, fantasizing about his eventual first kiss, parents separating, sneaking bits of protein powder into what little food his sister will eat in an effort to keep her from having to go back to the hospital.