Thanks for the input.
As time has gone on since the divorce, sis has become quite the partier, spending weekends in bars and drinking parties, etc. Nothing wrong with that, we have gone out with here a couple of times when we were "kidless" (which is rare). Its just not condusive to people with kids (aka US and former brother in law). It's not on purpose, but we end up spending more time with brother in law and less time with sis, strictly from the standpoint of shared interests. We have kids; she does not.
brother in law invited us and the kids to New Year's dinner tonight and we accepted. This morning, sis called and asked if we had plans, turns out she has to work tomorrow and isn't going bar-hopping, like usual, so wanted to get together. Of course, if we tell her what we're really doing, she'll blow a gasket. She knows we maintain contact with brother in law, but never specifics about it.
A co-worker overheard me call husband and tell him. Then several made several comments about how "strange" this family is. (We also spent Christmas at my ex-husband's parents' home. DEX has nothing to do with the kids, but I think its important they maintain a family relationship with their grandparents, aunts, uncles, and cousins on that side. Thankfully, DEX's family does, too, so we are very included. DEX is always invited and sometimes comes, sometimes does not - as was the case at Christmas.) I just feel like its the right thing to do for the kids, to teach them love isn't conditional or tied to family "boundaries".