I take it they still have a swing profile because her husband isn't sure he wants to give it up, and he needs her to partner up with him in a swing setting? If so, then I suppose they will just delete their profile if the time comes that her husband loses interest in it, given that she's already lost interest in it.
So, does her husband talk much about what he wants as far as a poly relationship is concerned? Is that something he might be interested in, and has he said as much?
Sounds like you have a great dynamic with their family and a hopeful chance that sleepovers with her might eventually be countenanced, even if that's not the agreement she and her husband started with.
I think it's great that you are high up on the cloud of NRE (New Relationship Energy), and I suppose it's possible that it might last forever, but you know, in most cases NRE tends to simmer down after awhile so just don't think anything has gone wrong with the relationship if that happens. It's one of those complicated chemistry things ...
So things are good and I for one hope they get even better.
Love means never having to say, "Put down that meat cleaver!"