If they care more about their daughter finding a house, and they have 3, then why don't they give her one?
In particular, the school systems that are around such homes. Location, Location, Location.
Secondly: those houses have tenants. They're not sitting empty.
Thirdly, for the young couple. Being tied to your parents financially basically makes them a slave-owner of you. When the parents drop by or use the yard to store project-cars, or refuse to fix issues with the house... the young couple is not in a position to fix those issues themselves without pissing off the parents. (Its not their house after all). So getting their own house leads to a better relationship between child and parent anyway, and should be preferred.
I’m sorry, but you don’t seem to understand the concept of giving. If I give something to my family, it is theirs. They can use it, lend it, sell it, destroy it.
I agree with parent comment, if they have 3 houses, buy one for their daughter. As in put her name on the deed. Either they care about her and love her and their grandchildren or they are selfish and would rather have 3 rentals then what’s best for their kids.
I'm very tangentially related to them. (Sister's husband's parent + sister's husband's sister). So I'm not really going to meddle with their life choices.
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But I'm trying to say that policies that help their daughter (even if it costs them a bit of wealth by subtracting it from their 3x houses due to looser construction laws causing more houses to be built and therefore lowering the prices of houses in this area) is something they likely will support.
You need a good school district if you want your child to growup smart. Retired folk having 3x homes in lower-educated areas is fine for retired folk. But that doesn't mean that those homes are immediately useful to a new family.
Building new homes (and cheaper homes) in the better school districts makes more sense overall. Some neighborhoods naturally become "retirement communities" but kind of a ghetto for education (ie: young couples try to escape those areas due to poor schooling).