This wouldn't even call for a "Bless your Heart" which is a veiled way of saying "you are messed up". She straight up needs a fucking verbal lashing for invading your sisters privacy. "God told me to" IS NOT a valid excuse. Stories like these make me glad I plan to raise my kids to be free thinkers.
This is so horrible, your poor sister. I'm so glad everyone is taking this as seriously as they should. I too love your parents, they are reacting in a very loving manner. I hope SIL is ostracized for this.
The part that pisses me off the most is that your sister was ALREADY feeling convicted and was talking to her BF about it. She was doing the right thing. Regardless of what happened in the past, she was doing what she felt was right to rectify the situation.
She wasn't blowing her BF at church camp. Your SIL is making it a WAAAAAY bigger deal than it has to be and is humiliating your poor sister in the process.
"Why would you ruin perfectly good peanuts by adding candy corn? That's like saying hey, I have these awesome nachos, guess I better add some dryer lint." - Nonny