Adopting one sibling from foster care

Hi, I'm new here and have some questions that I'm hoping someone can help a little with. My husband and I, and our two bio boys, just started in foster care. We took two sisters from a sibling group of 4 and had them with us for about three months. The older sister had some issues that ultimately affected the safety of my boys and so the two girls were removed. The younger girl did not adjust at all to the new home and two weeks later, finally got the okay to separate the two girls, and the younger one came back to stay with us. Things are going great and we just love her. There is a family that has expressed interest in adopting all four siblings. TPR will be official in January and this woman, who has only met the siblings once, and has two bio girls of her own, says she is taking all four kids. (7, 5, 3, and 9 months). She hasn't been fostering any of the kids. My question is, what is the likelihood that we could adopt the younger sister that has been with us all this time. It took some convincing of DCS to separate the girls for foster care. I understand every effort is made to keep siblings together and it is a miracle that someone has said they would adopt all 4. Having had both girls, and having met all of the kids, we see so much potential for the youngest girl to grow and thrive away from her older sister. We would love to be her forever family, but I'm afraid to even bring it up with my case worker, knowing they won't want to split the siblings up. And I'll add that she tells everyone all the time that she wants to stay with us forever. Any thoughts?? Thanks and cross posted.