Sadistic Kid

My family recently acquired guardianship of my 11-year old cousin Pete. His mother died last year of an opiate overdose. Before that, she had been addicted for quite some time, was having seizures and never held a job down. She had divorced her husband for embezzling money from her, who didn't see my cousin for about 7 years. Pete lived with my aunt, mainly taking care of himself. The house was never cleaned or cared for, and my grandfather would simply pay for all of their needs.
After she died, my uncle tried to regain custody without meeting my cousin, but we were given guardianship of him for a year due to the fact that he had already lost Pete in the divorce custody battle. My grandfather tried for custody but he is a hoarder, and his house is a mess.
Since being with us, Pete has displayed some odd behavior. He is understandably antisocial, but when he speaks he has an extensive vocabulary. He spends all of his time on his computer. After checking his history, we discovered that besides games and browsing doll websites (he has a huge interest in dolls, fashion, and things of that nature), he spends much of his time visiting sadistic websites. He likes to go through Wikipedia and look for descriptions of concepts like murder, 666, etc. My parents finally had the bright idea to install a parental control when they found he had visited a pornographic website about torturing gay males. Right after that he asked to spend the weekend at my grandpa's house, who gives him free reign and won't even consider some kind of safe program for him. He came home with a drawing of satanic symbols, and now has it taped to the door in his room.
That was sort of the last straw for me, which made me sign up for this forum. My father is completely uninvolved in Pete's life, and my mother is unable to control him. I have two younger sisters, 15 and 18, whom are very important to me. I don't feel like I am over-reacting in worrying about their safety. Pete isn't cruel to animals or deliberately mean to people, so I don't feel that it's sociopathy. We just need to figure out how to get through the next few months, until his dad can hopefully take custody of him.
Any advice on this matter, or insight into this general behavior, would put me much more at ease. Thanks for your time.