The only thing we promised to both kids is that we would pay for their undergraduate degrees. The $ cost was not part of that discussion. We said it would be nice if they won merit-based funding but the most important thing in their college search should be to find a college that suited their academic and social interests.
The only time a possible “fairness” issue arose was when our daughter decided to enroll in a professional school (MBA). But our son expressed no concern that we might be spending more on his sister’s college education.
Later on (in recent years) we also provided some financial support to our daughter. We told our son and he didn’t express any concern about “fairness” or “equality.” He knew that if he had been the one in need we’d have tried to help him out.
In short: there was no sibling rivalry based on the money we spent. They live in the same megacity, and they get along well. But even if there had been such rivalry or jealousy we wouldn’t have handled it any differently than we did.