What does this sound like to you?

Sounds like you were in a fugue/sleepwalking and had a very bad dream. Not as uncommon as you might think, and quite common in people of ALL ages with sleep disorders, or whose sleep is already affected by stress and/or depression. Personally, I think your sister was either pulling your leg, or telling you what you expected/wanted to hear abot monsters being in your room...you don't sound as though you could be talked out of anything but this being a real event (stubborn kid)...so she probably just said that to shut you up. But in my opinion, you had a dream..a very bad, bad dream. But if you want to believe it, I won't stop you.