Having your "own" money when in a marriage is bound to lead to division and rancour.
It is obvious that unless they do identical jobs, one partner is highly likely to earn more than the other. Although upon marriage the partners do not cease to be individuals, there needs to be a considerable commitment on both their parts which means their individuality must be forfeited to a degree. Marriage is (or should be!) a long term project. Throughout that term there will be ups and downs for both parties. What should happen, say, if your son-in-law found himself unemployed? What if your daughter became pregnant? Clearly a radical re-think of their financial arrangements would have to ensue. Your daughter has chosen to run a house which, had she been alone, she may not have been able to afford. She chose to do so because she entered into a partnership. My view (and it's only my view - not gospel) is that I believe it is the height of folly to exist as two separate parties financially when they have so many joint commitments. Single people usually do that (which is where the problem probably stems from as they spent a long time together before getting hitched). But they are no longer single and there is no need for them to behave as if they were.
What can you do about it? Nothing. It's for them to decide. You can only express your views, but I think you're on a dodgy wicket even to do that.