Take your uncertainty and make it part of the story. Dad is in a tizzy, wondering if Mom knows or not. She drops some hints that make him think she knows, then others that make him think not. He gets more paranoid by the minute (and more horny). In the end.... well, that's up to the author of course, but I'd have had her know it all along and she was just teasing him until the time was right for a threesome.
"Hush, darling. I knew all about it. Now shut up and fuck us both!"