Happened to me a couple of times. I guess alot of family members kinda know but dont say anthing . What my wife says when i was catch by her parents she said it was new pj's an acted really cool about it, which didnt arouse any suspicion. Act cool an give a simple reasonable excuse.

My brother walked in my house as I was napping on the couch, he freaked. He left as I was totally dresses to the nines!!!.
I later told him am mom ( I was 36 at that time ) that it was a sexual thing with my wife, like role playing . Just act cool , have ur SO also with you say it was on a bet of some thingand play along what you make up.

Use your amagination or just come out offthe closet an pay the piper. Nobody asks me I dont tell ,just be calm if you stay in the closet. jen ...... Let me know what you decide , maybe i can help more. good luck