L.F.
Awww. How sad. It is true, she should be getting Social Security payments. You might even be able to get retroactive payments for her. Sorry, I don't have any actual specifics.
That is wonderful that your SIL is able to help her little sis. It is a big responsibility. My mom died when I was 11 years old. I absolutely hated the fact that I had to ask my dad questions about sex ed and had to come to him when I needed a training bra and started my period. (He was very nice about it, but I just felt weird talking to him about it.)
I was lucky that in high school, I babysat for a woman who kind of took me under her wing. You might be able to take that role for this little girl and just let her know that you are available if she has any questions.
I never had anyone to teach me how to wear make up, talk to boys, or navigate college admissions. I had to figure it out by myself. Maybe you can help her with that when the time comes. Good luck. What a sad situation.
ETA: She should have received Social Security benefits (or her father should have received them) when her mother died. Now that her father died, she might receive even more money.