My husband has same thing with one of his sisters. Our way with her, is to just ignore her, when ever she tried to start trouble and she does that alot. His sister is important to, as in that she is his sister, but we don't "feed her" sort of speak.

Sounds a little like your husbands sister might be the same way, out to start things. She want to be noticed and her means of how to get noticed is to be an A#$. I would just ignore the whole thing, sounds to me like she wants to get to him and by responding, you just "feeding her needs" and she feels she won.

If you feel you have to say something, I would leave it at: "Although we have a different take on it, we thank you for your letter." And let that be the end of it. It will kill her that she didn't get to him and he proves himself to be the better person.