My husband and I built a luxurious house near ours for his sister and brother-in-law after we learned that they were starting a family. Our motive was admittedly selfish: As two men, we wanted to be part of a nuclear family and help raise their children. We are well off; our brother-in-law is a blue-collar worker, and my husband’s sister stays at home because of untreated anxiety. We agreed to pay the substantial annual real estate tax and not charge them rent.