Brother in law troubles!

So me and my partner have been together for 3 years. We’re due to get married September 18th 2021.

Since we got together his brother has been nothing but a problem. He has tried numerous times to split us up. He’s one of those, voices his opinion even if it’s not right or needed, kind of people. He’s the oldest and thinks he has authority over the whole family, even his parents. There have been many many fall outs between the family caused by him but everyone always uses the excuse of “it’s just what he’s like”. I’m the only one who’s ever stood my ground when he’s on one with me and that’s why he doesn’t like me. But I’m not one to sit there and take it like the rest of the family. I always just carry on and talk to him like normal because I don’t want to make it awkward for my partner.

However my partner is talking about asking him to be his best man at our wedding. Which I just can’t get my head around. I would NEVER tell him no, because it’s his day as much as mine and I’m not that awful! I just don’t understand why he would want someone that’s constantly tried to wreck our relationship to be his best man when we’re getting married?! And his only reasoning behind it seems to be because he’s the oldest (there’s 3 of them, my partner is the middle brother) he should pick him. He has a couple of really close friends that I’ve suggested having so he doesn’t need to chose between his brothers, because in all honestly I think his younger brother would be quite upset.

Other than that my in-laws are lovely! He just spoils it sometimes!