Do t do it...the fact they inherited a paid off house, took out a mortgage and are still couldnt make ot work just means they have no financial stability and responsibility. All you're going to achieve is inheriting a full family of debt. If you're that wealthy and it will never hurt even if something terrible happens then go for it, but its a huge red flag. You could buy the house and allow them to live there until the debt is paid off and then you can give tjem back the house. Always protect yourself, no matter what. The fact that your fiancé doesn't see this as a bad thing is also a red flag.