Hi there,
Sounds like a big mess. Since you are in the process of buying a house, I would check with the insurance company & find out if the accident would be covered along with the girls injuries. I would bet that the girls injuries would be covered, but that the car would not be, since there wasn't a 21 YO person with your daughter while she was driving. If insurance will not cover any of it, then you are stuck.
As for the girls, I would ground them both (for an undetermined amount of time) & make them either get a part time job or do extra chores to repay you for your out of pocket expenses. This would probably take them the whole school year & then some, depending on the cost of the damages & injuries. Take the keys away from them. I agree in not letting the 15 YO getting her license until either she has paid off her part of the damages OR until she is 18.
If insurance covers everything & raises your rates, then I would make the girls cover the copay along with paying the increase in premium until it comes back down. Once you feel that they have been grounded long enough & plan to let the 18YO drive again. I would make her pay for her part of the coverage, & if she can't afford to do that 1 month, then she can't drive that month.
Also, if the 15 YO gf is unable to do her chores because of her injuries, I would make the 15 YO go over & do them for her.
I hope this helps.
Best of luck & may God Bless you all,
C.