I read all your replies. If you "loan" that money to them, its fucking gone. So its a gift if you do it. It doesn't matter what they say or what mortgage fraud plans they come up with. Only give them the money if you are at peace with never seeing it again. Don't fool yourself into thinking its coming back, that will just make you mad and them mad when they don't. Because they won't be paying it back.
That said, I have no idea why they are so house crazy. By your own description their credit is crappy, they have no assets and their income is not impressive. Also by your own admission they will in a couple years likely have much better income, their credit may well have improved and they have two years to build up some savings. And your sister in law is 21. 21! Why does she think she should have a 280K house right now anyway? Whats the big hurry? In two years she'll be the ripe old age of 23 which is still pretty damn young to be owning a $280K house. I see no reason to pity her, its not like she's getting thrown out on the street. And if she wanted a house right now she should have married one of the plethora of 33 year old men that are better established instead.
Its also pretty hard to lend money to some one with no money so they can live in a better house than you own. Didn't you say you were looking for a more affordable house for you and your wife?
Frankly, I'd be rather annoyed I was even asked this. Its not like its for a lung transplant or something.