Girlfriend brother issues

I have a question, I'm in a relationship with a absolutely incredible girl, this girl means the world to me, her whole family likes me except her brother. Why? Because I tried to witness to him. I tried to witness to my girlfriends brother one night telling him and warning him not to go out that night and do stupid things but he went anyways, he does not like me because he is arrogant and wants only what he wants, he is extremely lukewarm, and believes in the death bed philophosphy, which means right before he dies he'll ask for forgiveness which doesn't work. Well how do I handle this and go about this? I don't know how to deal with this? The girlfriend is on the side of her brother and told me not to witness to him because she is afraid he'll leave her family. How do I handle this?