Hope for the best, but prepare for the worst.
You could think about creating a false sense of bad blood between the two of you, for the benefit of your wife and/or sister. Just a comment here and there. "We were talking about x and I said y and I seem to have really offended her". Then, if she does try to out you, it's your word against hers and you've set up a motive for why she's behaving like that.
You may also want to get the rest of your "punting house" in order. If the wife/sister smells smoke, she may go looking for fire. Make sure there is nothing else to find.
In all likelihood you'll get away with it, but I wouldn't be resting on my laurels.