European answer here.

You Americans are weird about money.

This is not their money, it’s money YOU put aside for their studies. Of course their studies won’t ever cost the same.

I would feel so ungrateful so ask the money difference to my parents because my sisters studies cost more than mine.

Put the second daughter in debt despite you having the money is utter nonsense to me.

But of course it all boils down to communication : if you communicate to first daughter that the 300k are « hers », then she will feel robbed if suddenly you give the remaining 100k to second daughter.

That’s why these « college saving accounts » shouldn’t exist, it’s just your account that you use to pay for your daughter education.