YTA - This is a problem stemming from a bad deal you've set yourself up for. Having your ex save money for your children's future whilst you pay current expenses is a bad deal. Both of you should have been paying half of those expenses so that you could have saved (more, choosing not to save some money is moronic when you have the option) for both your own children and you new family. You would have had less of an issue in that case.

Your step-daughter isn't entitled to your ex's money, but as far as I'm considered you are partially responsible for your stepdaughter's education ever since you got married. You could have seen this coming for at least the greater part of a decade, but chose to not do anything about it. You are being callous about it and have been for at least seven years. It wasn't your ex's job to think about your stepchild, but it was yours.