Child support is calculated only on the bio parent(s)' income but determining college expenses can and usually does include spouses and sometimes even non-spouses if they are proven to be supplying 1/2 or more of the minor's living expenses - such as our case. I didn't know this and was pleased to find he'd supeona the boyfriend's payroll & bank records of the exwife to show she doesn't pay household expenses for stepdaugther but the boyfriend does AND makes more than we do - thus ruining the ex's ideas of my husband paying more than 1/3 of the college cost for his kids.
The atty said child support will stop when stepdaughter graduates high school and then we'll just be coming to a decision on college expenses for both stepkids. Interestingly, all aspects of the court order (insurance, medical expenses, etc) will be cancelled along with child support unless we choose to use them as negotiations with college. That was good news too.
This atty disagrees with IL law forcing divorced parents to pay for their child's college so he fights for less than the state mandated 1/3 for each parent & child & feels things like both parents involved in decisions, minimum GPA, etc should be included as well. He thinks in our case, we should continue to pay for his son's college (lives w/us and we have so far w/o help from the ex) and his ex shall pay for daughter's college since she lives w/her and call it even. Oh and only until stepkids drop out of college or get an undergraduate degree. None of this paying till they get their doctorate degrees either. We'll see what happens.