I was a straight natural until I was 20... and then I relaxed my junior year in college. Before I started going to salons at 15, my grandmother would press my hair. At 11, I had no idea how to care for my hair, nor did I care to because I knew my grandmother was coming over to do my hair. All I knew was that my hair went from 8 braids of natural wavy curly hair to straight hair down my back. At that time, the only person that cared for my hair was my grandmother.
My point in telling that story is to say, that we can't really expect a 11 year old to properly care for her hair. I know some of you cared for your hair at young ages, but I question whether it was properly cared for. MsChelle, I think that she will definitely need your guidance and assistance. I don't think you should feel guilty, I just think you should continue to help her maintain her tresses.
You know the family better than we do, and the impression that I get is that neither the father nor the young girl will properly care for her hair, so continue to help her and show her ... and hope that she will take interest in maintaining her hair and apply what she learns from you!
Wishing for the best!