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.