If it's something small and innocent, don't worry about it, we all have out secrets, even in marriage.
If it's something really important, that might hurt your sister, or that would cause her anguish to learn about, then you have to find a way to get it out in the open.
If it were me, I THINK I would start by talking to the brother-in-law and telling him that you know, and that he has to correct the matter and then tell your sister. It will hurt your sister for a while maybe, but it would hurt a lot more later on to find out that you knew too and didn't care enough to do something about it.
Either way though, there is always a chance that she won't take it well, and both the brother-in-law and your sister will end up mad at you. Sorry, but it just gets that way sometimes.