AZOI.IS, as a mother and MIL, I certainly understand how you feel. WE too are the givers in every scenario and it isn’t easy or pleasant at times when it is always one sided. However, on the other hand, b”h we do what we can and we have the pleasure and satisfaction knowing that Hashem has given us enough to be able to help our children and gift our children when warranted. So first and foremost said B”H that you are able and willing to give.
Secondly since it seems you missed my point earlier. You can’t protect your daughter in the event of a future divorce unless you had the foresight of having her sign a prenup. Just as you have heard terrible stories of children losing their houses to their husbands, I am sure that you have heard husband’s holding the “get” hostage for huge amounts of money. So in that sense you really can’t protect neither YOUR investment nor your daughter from what might happen in the future.
Therefore you can either give the money as a loan or a gift to your “children” with a full heart and not cause any machlokes or problems between them. Before we even bought our first house, we moved into a larger apartment and we couldn’t afford to pay for the move, the painting and the carpeting. Both my father and my FIL lent us money. When we had the money to repay them both my Father an FIL refused to take the money back. Their loans were NOT equal but each did what they could.
I have “lent” my clients money (over the years this has added up to quite a bit) never expecting to be paid back. To them it is a loan, in my mind it is a matanah. If they give it back, I am surprised because I usually have forgotten about it, but they didn’t and it makes them feel good. If they don’t I am not worried because I know that Hashem will repay me one way or another.
So my advice to you is if you can’t do it with your whole heart then now is not a good time to do it. If you can hear and understand what I am telling you then decide whether you are lending them the money or giving them the money and include both the children, then forget about it.