Oh Bethany, I am so sorry. I would be livid and agree with Overwhelmed about the approach. It is unacceptable, but I cannot imagine that even if told that information that SIL is dumb enough to share that she knows.
And for the record, you do not have to explain to us why you have shared a bit here and there. We all get it. Most of us sometimes share slightly more here than in real life with friends and social circle, because this is a group of people who have totally been there.
