Is my sister coming on to me?

She doesn't know how to be family. You should be able to wear a bathrobe around the house 24/7 if you want because it's all family around there. Families should be close enough to know that none of them are coming onto any other and if one does, he or she shall be shunned. She may have been curious about you in a non-family kind of way. If she was curious about you that way, then she hasn't accepted the fact that she's your sister.