My sister got a divorce over 10 years ago.
I think for your nephews sake you should be cordial.
I agree with Anna. The last thing kids want is their parents fighting like the couple in "War of the Roses". If their aunt is involved, it makes it even worse. Tell your sister she needs to think about the kid(s) and stop acting so immature.
Unless he has abused her or has done something really bad, you and your husband have every right to talk to her ex.