I hooked up with a random guy at a party. He's a total jerk, so I intended to never see him again. But he's my brother in-laws nephew and I see a lot of him. I don't know how my brother would react but this guy is trying to get closer to me! Should I tell my brother?
"he's my brother in-laws nephew" In many cases, people like that aren't that close, and in others the relative realizes the flaws of the person. I'd say you have a pretty good chance of your brother-on-law taking your side in this problem, but I don't know.
I'd say if you think you can handle it on your own, do it. If you tell the guy you don't want to be around him anymore, he should get the message.
If he's so think-headed that he doesn't, then you need to seek someone else out.