So because there were so many games with lower prices my sister, dad and I got a lot. A few are M rated like DOA 5, Assassins Creed 3 and Resident Evil 6. I know for a fact that dead or alive should be kept under lock and key away from him. And so should the other games, but I fear he's curious , there's been many times where we would find him sneaking on playing games like red dead redemption or call of duty and swear my dad allowed him too , but we find out he didn't. Other times we would find him watching gameplay or cinematics on YouTube and since I have things of my own to worry about I find these out by accident or pass by and see he's watching stuff he shouldn't. Ontop of that he keeps insisting certain games and movies are no harm to him (he's 11 I'm 15 my sister is 18) and my sister and I keep telling him to wait to watch or play these movies and games because you'll understand it more. It's frustrating because I know now why my sisters would prevent me from watching certain movies when I was younger and now I'm not as corrupt as I could've been , and he totally thinks he and I are on the same level , like he would say how come you can watch Sierra (my sister) play and I can't? How come dad lets you watch this and I can't bla bla bla.

My question is
How can we find out what exactly he's been watching on YouTube
Is there any content in the cinematics or gameplay of any of these games we should be concerned about ( in a video of RDR there was a scene much like GTA when you pick up a prositute)
What way can we prevent him from getting his hands on these videos or video game

Oh and how can we prevent him from getting his hands on certain movies? My sister and I use 1channel and he found out about it and nower days I notice him bragging about the movies he watched ( that are clearly on the boarder lines of R but still rated 13) and we tell him not to but he would sneak on at night or make a tent over his computer and watch it anyway