I wasn't going to respond until I read the advice given you. I also see both sides, but disagree with Cheryl, I think that being around people that have different morals can be good for kids. It teaches them that even though we may disagree with a persons actions, we can still love them because they ARE family. I had to be openminded because my oldest sons biological father finally decided he wanted to be a father when my son was 6-he had visitation rights, so what could I do-and his morals were definitely different from mine.
The world is made up of people who don't think the way I do, and I feel it would be a terrible disservice to my children to teach them that if someone does something I don't like, I can just pretend they don't exist! That's not even reality! I hope there aren't any grocery store trips planned, because that store is sure to be full of people who MAY have different morals.
I do agree with cheryl about not forcing my kids come in physical contact with people they don't want to, but to me that's completely different.
Very long-winded, and not really even advice!
I guess all I can say is this:continue to invite them both, it is their decision to come or not. If your sister comes and starts making a big deal, then lay down the whole my house/my rules thing (it is YOUR house, not sisters after all.) My favorite house rule is: 'if you don't have anything nice to say, don't say anything at all'--it just may work :)