As awkward and unpleasant as this must be for you, I think you need to realize that your husband is trying to have time with his kids, and may not know how to make more of the situation than it is. He probably doesn't want to push them for more interaction out of fear that they'll stop coming. It rather seems to me that they don't particularly want to be there, so they just bide their time. The 18 1/2 year old has a choice as to whether she comes to stay, but the 16 1/2 year old does not. Perhaps his big sister comes out of solidarity.
My advice, since you ask, is not to curtail the visits, but rather to try to help your husband understand that he can assert some will and have them come out of their rooms and do something with him, and you, if you want to be a part of it. Do you ever go anywhere as a family, out to dinner or to the movies? Museum trips, or whatever the kids are interested in, maybe a ballgame if their more sporty? It just sounds to me like they don't really know what to do in the situation, so the adults should take the lead.
It sounds like a difficult situation, and an uncomfortable one, and you're in the middle. I'm sure you don't want to push yourself on the kids. I really hope your husband will try to get everyone out & having some kind of fun next time they're with you for the weekend.
I wish you & your family all the best.