Why does my girlfriends brother hate me?

I don't know why it could he that he doesn't like me dating his sister. he is always giving me that tough guy cold look when I stare at him. he was in my geometry class and I could just feel him staring right at me and when I turned around he was staring right at me. he is always bumping into me in the hallway trying to knock me down. today I took his sister out to the carnival and I offed him to come not thinking he would say yes. so I came over to her house so that we could leave and he tried to hit me with his car. while we were at the carnival I would go on I couldn't even go on any rides with he because he always cut infront of me in line. I won a inflatable animal for her and I was going to give it to her and he poped it I tried to put my arm around her and he growled at me like a dog . when we got to her house he went inside and so I. leaned in to kiss her and he opens the door and tells her its time for dinner. he locks the door and I stood their pist. He was staring at me through the window. He even talks to his dog and telld it to attack me