We do, indirectly. My sister not only keeps in touch with her ex, they live less than 2 miles apart. They have 2 kids, both now married, one childless by preference, the other with 3 young kids all under 9 yrs. Family events are held at his house because it's the biggest and best for entertaining.
Both my sister and ex have dated, and the tacit understanding is that their dates, serious or not, are always invited to family group events.
In fact, there are times my sister and I have not spoken to one another, but I still kept in contact with my ex-BIL. He's a great guy, very sweet.
My DH is still friends with his ex-BIL as well. We owe our house to him; he offered us a partnership back in 1989 to buy it and we took him up on it. Ended up taking the place over on our own after he ran into money troubles. It's how we wound up with a small home that was custom-designed by us on the interior and has pretty much everything we want (including a ton of closet space, woohoo!).
That ex (again, DH's side) worked for a while for a solar energy company. We asked him in 2014 for any tips to make sure we were getting a good deal (from a different company), so he came over on his own time, checked out the sample installation kit and price quote. He confirmed we were getting a good deal and that the company was using quality brackets for the solar panels. He lives very close to us and has helped us with plumbing and furnace problems, too.
Admittedly, our friends think it's hysterical that DH and I are on better terms with the two ex-BILs than our own sisters!