I actually had this same problem w/ my sister and my cat Ali. My sister was the one that bought Ali from the pet store and brought her home, but I was the one to take care of her. When my sister moved out (I think she went to college...I can't remember ) she left Ali with me (I was 14 or 15 at the time). When my sister came back from college about six months later she still left Ali w/ me because she went to live w/ my mom. When she finally got her own place (about a year later), she wanted Ali back...I had to give in because my dad said that she was my sister's cat Needless to say I was very upset. It was the final straw for me though when my sister's husband threatened to declaw her ...thankfully I won in the end and Ali has found her forever home w/ my husband and myself. Hopefully all will work out with your family too.