This just happened to me last night actually. So my boyfriend and I spend a lot of time at my brother's house. I practically live there. Anyway, my brother's girlfriend also lives with him. At this point in time my brother was at work and my sister in law was at college doing something. The boyfriend and I had just went out to eat for dinner and we come back to find the house empty. So naturally we start making out and things get heavy. My phone rings, so I go and answer it, it's my sister in law asking if we wanted anything to eat. We had just eaten so I tell her no. So I hang up the phone and jump back in with my boyfriend. I hear a little crackling noise and couldn't figure out for the life of me what it was. Turns out it was the door unlocking and my sister walks in. Damn she got there fast! We both pulled the cover over ourselves and she just walks in, with her food, sits down on the living room couch oblivious to what just happened starts eating and watching tv. We sat there naked under that blanket for about an hour until we could manage to put clothes back on without her noticing.