Help for School Story

I am quite stuck about what the plot should be for a mysterious short story we have to write for school. I really have no ideas what so ever and any ideas would really help.
My teacher said it should be something that plays with your mind a bit and I want to avoid stereotypicical characters. PLease help me if you can.

Mysterious? As in a mystery or just a suspenseful and dramatic piece of writing?

Whatever it is, start off with a character and make him have a problem. That always focuses a story.

So... um... who is your main character?

Em mysterious as in suspenseful and the sense of slightly scary but not gorey.

My main characters are so far a younger sister and an older brother, the sister in her younger teens and the brother in his older teens. She is quite quiet and shy whilst he is more confident but not loud.

We had to do these. Find a fear to play on, something you're afraid of, too - like Edgar Alan Poe and being buried alive, and go from there.