i really don’t want to be my future brother-in-laws groomsman.

You don't want to do it even though it's moreso for your sister rather than him? Can you be a bridesman instead? I would just suck it up but I also understand that being a part of someone's wedding party comes with certain expectations. If you really don't want to do it maybe say moreso you've got a lot going on in your own life right now

It is not her place to choose her partner's wedding party. If she really wants you in the wedding party then she should ask you to stand on her side as a bridesman or man of honor which is becoming more common now. But do not be a groomsman for someone that you do not know well or like. The wedding party is supposed to be the nearest and dearest and the couple should be choosing their own "nearest and dearest" and not trying to choose this for each other. I would explain this to her and then let it go. Don't engage with her if she tries to bring it up again and change the subject. Hopefully she'll get it and move on.

So I had a situation where my husband and I were engaged 2 days prior to his sister. She asked me to be in her wedding before either of us had a venue. At the time, I wasn't really thrilled with my husband's family's behavior. I said no. I felt mildly bad about it. I knew that I couldn't commit to her wedding without knowing any details of either upcoming wedding. Frankly, I didn't want to spend time during my planning process focusing on someone else's wedding. I knew I wouldn't be a good bridesmaid and likely heighten tensions. Looking back, I absolutely made the right decision. I'm in a much better place with my husbands family now. If I had been immersed in her process I likely would have carried my resentments into the present. Luckily, I had time and space to find acceptance. Go with your gut.

I don't think your sister should be delegating groomsman. It's his choice. I also think you can say no. There are other ways you can be involved, such as a reading. It's a huge commitment and it may cause more issues along the way. Good luck.

Your sister doesn't get to pick the groomsmen that's only for her husband to do. You can tell her it was very thoughtful of her to think of it but you're not comfortable with it. If she takes it personally that's on her you can't change her personality.
As a side note I know exactly what you mean by he's the embodiment of Reddit and I am sorry your brother in law is like that! Also you made me chuckle, thanks for that.

View Quoted Comment Just as a bonus question to you. Do you think I should keep the feelings I have about him to myself or should I tell my sister?

View Quoted Comment I think in the current situation and what you've said about her, she would react poorly.
If she gets upset about you being uncomfortable being a groomsman you could try saying you always thought shed marry a more humble man and you just don't mix well with him. Reroute the focus to how you both don't get along.
I find it hard to believe this type of guy hasn't said some negative things about you to her before, so hearing it from you might make it more clear this is a personality conflict and not about her. Finding some concrete and specific examples of her fiance being unkind to you would help if she brings it up.
My overall concern would be your sister flipping out and uninviting or even cutting you out of her life, which doesn't sound like a goal you have. So you'd need to emphasize the fiance's words and actions as the problem over her.

I think it's fine if you politely decline if he asks you to be a groomsmen. Like others have said, it should be his choice of guys anyway.
I completely relate to not backing a siblings choice to marry. My brother did that to us 10 years ago....we've all tried to keep the peace in not really approving of his wife. It's hard sometimes but it's not worth losing a relationship with my brother over telling him I loathe his wife.

There is no need for brothers or sister of the bride or groom to be in the wedding party, most especially not on the side of their new in-law. Say that Groom should have long term best friends of his, stand up with him, and you and he have not been close before now. ( Don't have to say, and never will be ...). And stand your ground, passive resistance but no argument, til she gives up. It may help is you volunteer to do something, like escort your mother or and grandmother to their seats before the processional. A common family, not wedding party, task. Or if you have family who need to be picked up at the airport, or older family who do not drive, picked up at home. Something that supports your sister's planning, without dealing with him . If you have very good printing or writing, sister may appreciate help addressing invitations, envelopes for thank you cards, doing escort cards or place cards. Whether you want her to marry him or not, she is. That you must accept, just as she will have to accept your choice of a person for you. But supporting her planning stuff, not dealing with him, should be enough. Some families nearly merge at marriage. Other in law's see each other rarely. It is not sister's right as bride to make you and groom close. She needs to drop that. And also, respect that you do not choose to be a groomsman, no reason given except, the two of you are not friends of long-standing. And he has other guys who are.

View Quoted Comment Yeah. Okay. I think what makes it harder is that he lives in my house. My sister moved him in without me, or my moms, or my dads permission. And that makes everything much more uncomfortable. When I told her I felt uncomfortable that he was living there she got angry at me pretty badly. And all I said was literally “I feel uncomfortable with him being here.”

View Quoted Comment He lives in your home and speaks to you like that? You are a nice guy.
My sister and her fiance would be homeless if they tried that!

It's totally ok to decline being a groomsman, although I would keep you're feelings to yourself about her FI.

Honestly, I would hope my brother would suck it up because my fiance is the man of my dreams. You might not understand what she sees in him but by being around him and maybe just getting to know him better will help you understand. She wants you to be a part of her special day and that's so kind of her. Hope this helps! Good luck!