My sister asked for and I don't know how to respond!?

You both need some time away and your sister needs counseling. There is no way under any cirstances you can confuse your sibling for a guy you want to sleep with. I think your sister ha been very confused about men especially the ones in her life and now feel that is the only way to feel loved. I think you need to confront her in a few days and tell her all the reasons why her actions were wrong but you love her as your little sister and could not hold anything like that against her. I think she still needs to see a therapist. good luck.