What to do about my rude sister in law?

Don't stress about this!! you said they ignore you at family gatherings..this is a good thing..let her ignore you..and if your brother in law is telling you it is her and her attitude, there again, don't worry..they probably won't be together for very long..And you are right..dont do anything that might harm your husband's relationship with his brother..Just let the relationship between she and your brother in law go, and odds are, this time next year she wont even be in the picture..be sweet and loving, as hard as it might be..let her be the ***** in the relationship..