I'd be kind of glad that your sister-in-law took the "lead" and contacted your kids and I might even send a little note saying the kids enjoyed the card and you're glad she'll be there for them (if you liked that sister-in-law :-) It's unfortunate but you're not going to be there anymore when they're with that family and it's nice that the sister-in-law is "paying attention" to the kids themselves now that they're growing up and are part of this big thing that is hapening in their lives?

But something friendly might elicit something friendly in return and will at least let her know you're "watching" and aware. You and your husband are going through the divorce so carefully but the rest of the family are in the dark as to what it's all about and how it's working and only have their son's/brother's point of view. I'm sure some of the women realize how odd/painful it will be for you (and them) and perhaps agree with you on your brother-in-law's ex-wife's treatment and would love to have an "excuse" to write you without having to "start". They may even think you hate "them" since you are divorcing their brother.