I think what you're looking for is a way to make it Less awkward between you and your sister, it's gonna be some kind of awkward for the rest of your life if this is true.
I'm betting it's not, who the fuck goes at it with some random you can't even see in a dark room???? I mean how did you know it wasn't a guy or something, how did you not recognize your own sister??? At least she didn't start blowing you or something. Maybe she wanted you, ever think of that, she must have seen something when you opened the door right.
This is just a huge steaming pile and I don't believe it for a second.