How to get my brother to stop sleeping in my room?

Im 17 and my 11 year old brother wont sleep in his own room. he insists on sleeping on my top bunk as i have bunk beds. i could understand if it was every once in a while like when mum and dad are both at work late. but he hasn't slept in his own room since at least Christmas. dad tends to sleep in there if he is having a bad night as he has sleep apnoea so doesn't sleep properly and mum sleeps in their room. my brothers room mostly stays empty. iv'e spoken to my parents and they think he should go back into his room but if we ask him to go back he says yes and then goes into my top bunk anyway. i don't mind him in there but its the lack of privacy i hate as he just walks in during the day and doesn't knock if the doors shut. what can i do?