I mean, do you have a relationship where you can talk to your family members seriously about housing?

Also, your phrasing makes me pause, the whole point of capitalism is to own as much private property as possible. So no, owning private property is not unethical. Owning property under capitskism is actually ethical and encouraged.

I’ll assume ethical in a broader sense, and say its part “no ethical consumption”. The best option would be to just give the deed and mortgage over to the tenants. Next down would be to have them pay just enough to cover the bills as if they were owning the property. Just above the line of completely unethical I would say is making a living off being a landlord, but being the best housing manager for the tenants. When things are broken it gets dealt with immediately, if tenents need a month give them one, be nice and actually build a relationship with the tenants.

But honestly there is very little for you, personally and individually, can do about the situation. Just try to talk them into being better landlords OR go to their tenants and help if they want to make a tenant union against your family members.