It’s not reasonable to expect colleges to contribute to the household expenses of a married couple just because one of them is a stepparent either. Why should colleges give more money to a student who has 3 or 4 working parents than they give to a student who has only 2?
If the stepparent didn’t want to contribute then the student could look for schools that would be affordable based on just the mom and dad’s incomes. It doesn’t sound like your teen is in that situation. I’d set a budget and apply to schools you know will be affordable (based on total COA and the assumption of being full pay or full pay minus guaranteed merit), then add a few financial reaches. You may not be able to afford all their favorites, but most parents can’t. If you have 3 people with good incomes who are willing to pay, I think you’ll end up with some great options.