AZOI.IS, your kids are NOT the only ones who pay rent to pay off the Landlord’s mortgage. Azoi is is for most young couples until THEY can afford to buy a home. Owning a home is expensive and one needs to be able to handle that on their own. It is NOT enough to be able to handle the mortgage, they also need to hand the insurance, taxes and any repairs and surprises that pop up. Are they looking at a one family or two family? Are they counting on additional income from a tenant? What if that rental is empty for a while? Will they be able to handle it or will they be coming back to you for help?
Honestly, paying rent is not the worst thing and really that should NOT be your problem. If it bothers you too much then this is really your issue and not theirs. Again if you don’t feel comfortable “gifting” them the money then tell them both, I can’t give you the money at this time, but I can “lend” you some money for a downpayment. In that case both of them will be responsible to repay the loan and you are not singling your child out over your in-law child. You will have time in the future to decide whether or not to accept the repayment. If you accept it, you can then turn it into trust funds for the grandchildren or find another way to secure it for your daughter or grandchildren.
Health, if you need me I’m here, but if Hashem has not sent you my way, I am pretty sure you are not desperate for my help.