Under siege in my room

So right now I am trapped in my room, locked up like some naughty child trying stewing on ill-will and resentment.

Basically, my house mate and I have an agreement- we don't allow other people to stay- he's cynical and grumpy, and I have a phobia of strangers. My boyfriend suffers this, and we are all in agreement of the exceptions.

I am scared stiff of my housemate's sister. She is, in short, a bitch, and any time she's been here in the past she's paraded around the place, eating all our food, hogging our bathroom, etc, and telling me she has more right to be here than I do, since her parents helped my housemate with the bond. No matter that she has a house of her own, and that I am the one who pays the rent...

Last time she yelled at me I discussed this with my housemate and we agreed that she wouldn't stay anymore. And so what does he do now but bring her around at some wee hour of the morning, to dump her stuff all over the house and stay as long as she likes. It's already midday, and I'm too terrified to leave my room or even make any sound because she's still in the house.

I mean, I know this is partly an irrational fear, but we had an agreement, and I think I should have the right to feel safe. I would do the same for him.