Should we get involved or not?

Of course, you should definitely talk to your husband first. If he doesn't have a problem with it, then I would definitely do it. But, you also do want to have your husband talk to his brother and find out what's going on. From what you said, it sounds like what your SIL said is the real situation. But you can never be too careful. And you also don't want to take a chance of hubby's brother getting mad and coming to your house and starting drama with SIL. Tread carefully on this one, but do you what you think is right.