Paying the Bills

Hello. I placed my son in September, and the afamily agreed in writing to pay all realted medicl expenses that my insurance did not pay. I sent them the, hopefully, last bill at the beginning of January, through their lawyer, and they are now denying that they will pay it. I sent a copy of the letter to the lawyer, my addmission paperwork (stating that the amom would be responsible for all related charges; she signed it), and a copy of the bill to the hospital, AND spoke to someone at the hospital, and they have now sent me another bill. Any previous bills have been paid within days, this has been over 6 weeks. My question is...is it worth just paying this bill and being done with it, or trying to go after the afamily for it? They have already cut off the contact they promised, so I am now thinking that if me paying the bill will just end this disaster, I should do it. Please advise. Thanks Ress