Please help friends sister?

When I went to my friends house I whent to my friends sisters room to get a game, and in her closet I found some vibrators and the door to the bathroom was open so I went to ask here about them and she was in the tube playing with herself so I ran out and went home but afterwords I thought about it and I got very hard but the thing is she's only 12 ! So what do I do? And is this normal?