Definitely leave it alone. The guy obviously has no respect for you or your sister and his marriage. If she chooses to be with someone who can be so disgusting and disrespectful then that is her choice but do not bring it up again.
I am not saying to sweep it under the rug because it has already been exposed and clearly discussed between he and your sister. The fact that he said it to you doesn't mean you should be involved. If you were a complete stranger I'm sure your sister wouldn't care to talk to you since the comment came out of him, know what I mean?
Don't rehash on it. Continue your relationship with your sister. The only thing I would personally do (having been in similar shoes myself) is to avoid all form of contact with him. She is family, not him. If you don't need to see or speak with him-- then don't. Family get togethers are unavoidable but aside from a polite "hello" and "goodbye", there shouldn't be any communication that will bring up feelings of insecurity upon your sister or put her in another uncomfortable position like he did recently. If it happens again it will more than likely out a strain on your relationship with her and unfortunately she will probably avoid all gatherings where she knows you will be present.