Yep, have and would do it again if we needed to.

IL's loaned us the deposit for our first house. We've since paid them back. They also loaned BIL money when he was starting up his business. I don't know about SIL but I'm sure if she needed it, she'd have been given it. No angst... they loan, they get paid back. That way it is fair.

My parents are a bit different. My sister, BIL, niece and nephew live with my parents. My parents put on a massive extension ($150k+) so that sister could move in there. She pays "board", which means it goes towards the mortgage and bills. Lots of their other expenses are run through the family business.

And it bothers me. I love my sister but I sort of see it as a "free ride" - they have no plans to move out, now or in the future.

And I can see it getting very messy when my parents pass away - the house is in their name, therefore I am entitled to half of it. According to their will, anyway. What kind of a sister does it make me if I kick her out and sell the house? She's going to have to get a mortgage and buy me out... messy messy messy.

ETA: in the right situation, I see it being a wonderful way for parents to be generous to their kids. If this is going to help you, go for it