If he was caught LAST YEAR, then he was told before that p 0 r no behavior was wrong,...NO? He knows not to treat his sister like that,..and you TALKED to him? (maybe a little lenient!!!!). Cut his internet, smack him on the head, etc., no nice talk.
What has been done in that year to stop this?
He will keep getting away with whatever he can if you only TALK. The source could be anywhere, interrogate him.
What are you going to do when he's 11 years old, 15 yrs. old, etc.?