How to remove a bad influence from my boyfriends circle of friends?

Sorry this may be long. 🙈 Boyfriend acts different around ONLY one person and it’s his cousin (by marriage). Cousin is a freeloader, wants to drink/drive, and party all the time, has no real motivation to grow up and make something of himself. Cousin even shows up to my house unannounced and doesn’t say one word to me while he waits for my boyfriend to get home. Just disrespectful as hell. We even let him have a car for a few months, which he stopped making payments on (we had a contract), caused thousands of dollars of damage on.. and my boyfriend won’t hold him accountable and thinks his cousins shit does not stink. I don’t get it. I’m the one who had to pay for the damages. When my boyfriend is with him, he drinks and drives. He acts like a dick towards me. And normally he would NEVER do this as if he were to get a DUI, he’d lose his job and probably everything along with it. And he’s always extremely sweet to me and loving. But when cousin is around, he ignores me, and acts like an immature child. It’s a complete 180. No I don’t like the cousin after the car situation. But he has always acted this way around his cousin in the 6 years we have been together. No one else. All of his family and friends are amazing and we get along beautifully. Ok, point of this post is.. all of the candles have helped my life tenfold, but I cannot for the life of me figure out which one to get for this situation. For awhile, we thought cousin was moving 19 hours away for a job, but that fell through. Now he has a better job locally. Ugh. I just want him to not be in our lives. He causes so much disruption between us when he’s around and I’m over it. This is the only sour spot between my boyfriend and I. I’ve tried reasoning with my boyfriend, but he is saying family this, family that.. Do I do work on my boyfriend specifically, or his cousin specifically? I just want him to move/not come around us for a long time, if not forever.