Why does my girlfriends brother hate me?

my girlfriends older brother hates me it could be because I am dating his sister. he is always giving me that tough guy cold look when I see him. when we are in geometry I can feel him staring at me and when I look back he is. when I walk in the hall he is in his group of peole staring right at me and they are laughing and pointing at me. so tonight I asked my girlfriend if she wanted to go to the carnival and she invited her brother the one person that I didn't want going with us. so I thought that it would be a great time for me and her brother to bond but I was way wrong. I walked to her house. and her brother tried to run me over with his car but I tried to ignore that so when we were at the carnival and I couldn't go on any rides with her because he would always cut infront of me in line. I went to get some food and drinks for us and he slaped it out of my hand and told her that he would buy her whatever she wanted. I won a giant inflatable bear and showed him and he stabed it with his plastic knife and got her a giant teddy bear. I tried to put my arm around her and he growed at me. when we go back to her house I asked him if I had permision to kiss his sister and he said no and took her into the house and was staring at me through the window