What is the relationship of the 4 siblings? Will depriving any of them of their dream school create resentment if the budgets are not equal or is there enough maturity and bond among them to realize that “fair” is not “equal” without creating resentment? I think ultimately the decision needs to be explained to all 4 clearly with the rationale.
I think OP has set up the basics pretty well. The complications are the ex-husband’s contribution and the house. Although in a sense having these 2 sources from outside the current marriage simplifies certain things as far as “fair not being equal” is concerned. The easy one is that it should be pretty uncontroversial to allocate the ex’s contribution to the triplets’ budgets. The house is more difficult. If the current family has been paying mortgage, tax and maintenance in excess of any rental income, that certainly needs to be factored in. Also, unless that house was the deceased mother’s property, I would not consider it 100% the stepdaughter’s inheritance. How are other pre-current marriage assets to be divided in terms of inheritance (not just college expenses)? Is there an agreed formula there? Did the stepdaughter already get some distribution from her mother’s will/estate? Assuming that there is no clear existing roadmap, I think the inclination should be first fund the stepdaughter’s education from the house fund for the school that best fits her, with any remainder to be used to supplement the triplets’ budget if needed, but in a sense treat this as a “loan” where the stepdaughter may get more help with graduate school or the final inheritance. In a sense we are kicking part of the monetary dilemma down the road, but we are trying to maximize the college experience for the 4 siblings for now.
What does remain is if the triplets have unequal costs. I think the default here is equal budgets, unless the three of them can all agree on a different arrangement.