Don't want them, but don't want anyone else to have them

I'm new - posting for the first time(s) tonight. We've been fp's for about 6, maybe it's 7, mos. We currently have a sibling group 4 (full), 2 (half), 1 (full). The biodad of the oldest and youngest has requested to tpr. The biomom doesn't want them either other than to use them against biodad (they are her form of control - family has told me.) Anyway, neither parents want the children, but the grandparents do. But, they don't really. They had an opportunity to take them before they came to me, DHS had moved the parents in with each of them at different points in time, but the grandparents refused sole custody because they didn't want to care for the children. Now, they are pushing the parents to keep them - they are carting them to the visits, making them stay the whole hour, etc. The father hasn't shown for one, and that's fine b/c the girls are TERRIFIED of him. His name make them physically shake. Anyway, do any of you have encouragement or advise for me? The GAL told me the other day that if it weren't for the gparents, tpr would have already been started b/c both parents had requested it at one point or another. But, like I said the grandparents refused to take the children in the beginning. (When SW called me about the placement, she told me that they may come into custody, but they were going to try to talk the grandparents into taking them first.) The middle girl has a different father, and his mother has pretty much adopted us. We visit her regularly, and she is a lovely lady. She doesn't have the time to devote to a 2yo, she just wants a relationship with her, and that is fine with me.