How to deal with Swinging Sister..?

My sister and her husband have decided to have sex with other people. While my husband and I are not hardcore christians, we do believe that what they are doing is wrong on a million levels. We own a house that has 2 apartments, we live in one, they live in the other. Up until we found out what they were doing ( a week after their decision) we were all very close, and had a good relationship. Their kids were always at our house playing, we spent our evenings together, we were like best friends Now, I can't stand to be around either one of them, and neither can my husband. I don't want to judge them too harshly, so I haven't, but I can't bring myself to continue the way we were as a family. I have told them that I think what they're doing is wrong and believe that's all I can do. What do I do now? There is no way that I can be as close to them as we were, and I don't think that I'll ever be able to see them as 'friends' again. I cannot accept this horrible thing they're doing to each other as 'part' of who they are. Am I wrong to want to disconnect from them? Any advice or suggestions would be greatly appreciated.