With the oldest being 17 and from a different father, I'm not sure they will stay together. We went through a similar situation when my sister died suddenly and left 3 kids, from 3 fathers, behind. While we desperately tried to keep the kids together, bio-Dad of the youngest stepped up and was awarded custody while we retained custody of the older two. They missed each other terribly on a day-to-day basis, but I really think in our case she was well-served by being w/her bio-Dad... someone her Mom never allowed her to 'know.' All 3 are in touch, and 2 of them (oldest and youngest) see one another for a long stretch every summer. Middle child is another sad issue, and she misses seeing him.
It's so difficult with blended families. It sounds like Mom here cut ties with bio-Dads, and probably let the girls believe their Dads didn't love them/care. (much as my sister had done)... In which case, at the least I hope that if the oldest goes with her bio-Dad all three can remain in close contact with open ability to see one another. Anyone know where this Dad lives? Tucson? (hopefully)