Your problem seems to be with your boyfriend not the girl, and that's who you address this with, firmly, and strongly. My take on the "and sum" is mo' money since he obviously broke whatever agreement they had to repay her money. It has already caused a riff between you and him, and is escalating to your kids, so nip this in the bud even if he has to leave.

The lies and excuses should not be tolerated.