Well, personally I'm not sure when sticking one's nose in anothers relationship business became an okay norm. Especially since I'm betting someone "under cover" probably didn't get "walked in on" in a public area. However, it does seem to be the norm opinion. More importantly though, it isn't what you asked. You asked if you should forcibly out your sister. The answer is no. And if I have to explain to you why that's the answer, then you are a horrible person. And if you do forcibly out her, I hope she responds by doing everything possible to ruin your life in kind. Have a nice day.