Jess, if you love your sister but want to detach because of the drama surrounding her husband I think you're doing the right thing in meeting her for lunch now and then, and otherwise keeping your distance.
If you want to say anything, perhaps you could form it around that you love her, but the dynamics of your respective spouses means you'd like to stay away from the full family experience. It will be harder with a baby on the way, but your husband will probably not want to get close to them again. She knows this.
I suggest you look at how to assert yourself with people you want or need to have ongoing relationships with. If they were strangers you could be clearer, but they are your family and, like it or not, there's going to be some contact.
If you could learn to stay reasonably detached from your mother's dramas, it would cause you less anxiety. My own mother used triangulation and passive aggressiveness to control us (4 girls). At some point we decided not to pass on 'messages' to each other or try to fix things. It's left us in a stronger place, and I think she's been forced to become more open.