My cat Scooter honestly prefers my sister to me. I don't think he knows that he is supposed to be my cat! He is very very attached to my sister, he sleeps with her every night, goes to her first for attention etc.
I would keep Scooter with your sister. Don't feel bad about leaving him. He is with family and you can get updates and photos on a regular basis. Your sister will take excellant care of Scooter.