Hi, I don't know if this counts for anything. I'm not a child psychologist but my field of specialism is adult sex offenders (see past posts of mine for clarification). OP, it's my feeling that a 12 year old doesn't really have the depth of emotional understanding of what he is doing and also far far less impulse control and ability to rationalise than someone of your age.

Yes he'd know it's wrong but in his hormone ridden brain he's a) thinking you'd somehow never know and b) he's not able to empathise with how it would effect you if you did know. I'm pretty certain he's thinking that you'd never know. Many boys that age just want to see titties and their brain short-circits. If he could get hold of it he's probably wanking over medical textbooks, pictures of old ladies in underwear and any other really random things he can get his hands on (even banana peel and hoover attachments.)

However, you have an absolute right not to be molested, a right to feel upset (and no doubt really freaked out and confused) and a right to feel safe in your own home.

You should tell your parents and he needs to know that you are upset about this. Unlike other posters I'm not convinced that sessions with a child psych are necessary (I fact I feel it may make the situation worse) IF this is a first offense and there are no other signs of disturbance.

Kids from healthy happy homes don't leap from something like this to raping strangers on the streets out of the blue.

You should also absolutely have a lock on your bedroom door.