I don't think you have to worry about your parent in-laws feelings about cards and it being a "faux pas" to send to the siblings; your age/generation and closer proximity to you and not the parents mediates in a practical way in your favor.

Have your children keep in touch with their grandparents through cards/calls/visits and you can write personal letters to the siblings whenever you want; that's why it is called "personal correspondence" :-) One's own siblings do things together and exclude some, include others, etc. due to where they live; no reason that now you're not as much "family" as "friend" that you can't do the same! It's not like you're looking to date your husband's brother or anything; you just want to occasionally have a chat with a sister or sister-in-law!

The parents-in-law at this point are only "important" as being your children's grandparents; you all weren't that close and they're not your generation and you no longer wish to have ties to them except through your kids; that seems "natural" to me and I don't think they'll find it offensive (and even if they do, that's part of why you're divorcing their son; someone's attitudes/beliefs are at odds with your own! :-)