I'm a foster mom, so I will be answering this from that perspective.
This sucks. I'm sorry. It is so, so hard to parent other people's kids sometimes. You completely fall in love with the kid, and it's so easy to want to just forget that the biological parents even exist. You stand back and look at the situation and it can be so clear that the child would have such a better life with you. You just want the bio parents to disappear so that you can just carry on with your life. I know. I've been there.
But you HAVE to look at this from another angle. You have to see what would hold up in court. I can only go by the info you have given us here.
Mom was 15 when she was born. I know you don't think this should have any bearing, but this is a HUGE deal. Very few 15 year olds have the capability to care for a baby. You didn't go into details, but you said that the baby was raised by you and your mom. Not sure why she didn't leave the baby with her parents. Did she not have good examples from her parents as to how to be a good parent? She was a kid, a baby having a baby. There is a huge difference, maturity wise, between 15 and 19. Is it the ideal situation? No. But a 19 year old has a much better shot at successfully raising a child.
If Mom's attorney as well as the little girl's attorney both agree that Mom would stand a good chance in court of getting the little girl back, you have to understand that is what will probably happen if her mom pushes it. You need to come to terms with the fact that Sue will more than likely end up going back home to live with her mom eventually. It sucks, and is so, so hard to admit, but for your sake, and for Sue's, you need to. In the eyes of the law, you are Sue's relative, but not her mother. In the eyes of the law, you have been a wonderful caregiver, but not her permanent placement.
At this point, you need to continue to love and care for Sue, but accept the fact that her other mom is a big part of her life, and the big picture. You need to start working WITH bio mom, not against her. Start sharing things with her about Sue's life. Share pictures, drawings, videos. Meet at a restaurant for dinner. Invite her over for a visit. Let her really be comfortable with you. Because one of two things is going to happen; either Sue will stay with you or she will be leaving you. If Sue stays with you, you need to be able to truthfully tell her when she's older that you tried everything you could to help her mom. If she leaves, you want bio mom to respect and acknowledge the bond you share, and allow you to still be a big part of her life.
It sucks, and it can be so, so hard. But you have to be able to accept that as much as you love Sue, she has another mom who is going to be a huge part of her life.