Lonelyone,
I think your reply to your sister-in-law was awsome! If everyone else doesn't compare you to someone who is ugly, then she is just making a fool out of herself, and basically, were the bigger person who just "blew her off". In fact, i probably would have deliberatly did what you did. At least you have class enough to not look for ways to insult people. (which is what you would have to do in order to be able to keep up with weird comments like that.. which IMO is exhausting)