She will need any kind of documents to show that there's no intention of her staying there ie the followings:-

1) that she has a property in anywhere of the world except for the States, I guess if the lease is under her name, that's fine too

2) Bank account that she can afford to stay there without working, but in this case, since the sponsor is your bother in law, he will need to provide his bank statement and a letter that he will take care of all the expenses while she's there

3) Is she working? Cause a company's letter stating that she's currently employed and that she'll be coming back will help too.

4) Make sure she knows what she's doing there... from day 1 to the day she leaves

5) Anything you can think off that shows that she has other commitment outside of the States and that she cannot afford to stay there.

All these document is just to support you, sometimes, they dont even look at it, just ask you questions...