HOW DO I MAKE CHILD SUPORT FAIR WHEN I HAVE 50/50 CUSTODY OF MY DAUGHTER?

MY HUSBAND AND HIS EX HAVE 50/50 CUSTODY OF HIS 2 DAUGHTERS. AND CHILD SUPPORT HAS NEVER BEEN EXCHANGED BECAUSE THE GIRLS WERE ALLOWED TO GO BACK AND FORTH AS THEY LIKE. WE BOUGHT THE OLDER DAUGHTER A CAR AND PAID HER INSURNACE TO MAKE IT EASY FOR THEM TO DO SO. NOW THE EX HAS MOVED TO ANOTHER CITY AND THE YOUNGER DUAGHTER CAN NOT GO BACK AND FORTH BECAUSE SHE CAN'T DRIVE. HER MOTHER IS NOT FILING FOR CHILD SUPPORT SINCE SHE DAUGHTER LIVED WITH HER FOR MOST OF LAST YEAR. ALTHOUGH SHE LIVED WITH US THE WHOLE YEAR BEFORE AND WE DID NOT GET SUPPORT. NOW THAT THE YOUNGER DAUGHTER IS 16 HER MOTHER SHOULD PURCHASE HER A CAR SINCE WE DID FOR THE OLDER DAUGHTER BUT HER MOTHER IS REFUSING. WHAT GROUNDS DO WE HAVE TO STAND ON TO GET HER MOTHER TO PURCHASE HER A CAR AND FOR US NOT TO HAVE TO PAY HER CHILD SUPPORT IN EXCESS OF ALL WE DO LIKE PAYING CELL BILLS AND CAR INSURANCE. HER MOTHER ALSO MAKES MORE THAN MY HUSBAND AND IS MARRIED AS WELL.