Do not want my sisters bf at wedding

Is the date below your name correct? March 2019? If so an array of things could have happened by then, they could be married, they could have broken up permanently or any other variety of things could have happened. I'd wait and deal with this issue closer too. Either way if you tell her he can't come you will likely have to deal with the reality that she may not come too. You will have to decide if allowing her to bring him is worth it for her to be there.

This is her battle to fight, not yours. If he makes a scene its on her as he would be her date. If you really don't want him there anticipate her decline as well (though I don't agree with this, I'd always support my sister even if she hated ,y lover). Regardless, its your decision, just don't be hurt if sis says no thank you and cancels her flight.

Edit: WHAT MARCH 2019? Heck they could be married before then, or have a baby, or be broken up for good. Don't bring this up now. Even if the hotel is booked I'm hoping you haven't sent out proper invites yet. Wait it our until you really need to know. Don't rock the boat now and risk ruining your relationship with your sister now.

Your sister is probably well aware that you and your family aren't his biggest fans but she is (presumably) an adult and can decide on her own if he should attend or not. If they are still together at the time of the wedding, he should be invited. Not inviting him to your wedding is not going to break them up even though it sounds like it is hard for you to see them together.

I get where you're coming from and it's hard to see people we love in unhealthy relationships. That being said, are you okay if she declines coming because he isn't invited? There is a good chance that could happen, either because she feels insulted, or because she doesn't feel it is safe to leave him alone while she's at a DW.

She's made her choice to be with him and you have to respect her choices as an adult. Hopefully this will become a non-issue though. Your wedding is a long way off, in that time they could have broken up, gotten married, or hopefully gotten help for any issues he's having. Encourage your sister to help get him into counselling for any suicidal thoughts he's having, even couple's counselling may help push him towards getting the help he needs. Depression and suicide are serious issues and he needs help and support, not to be isolated. Pushing them away isn't going to help anyone in this situation.

Do not, I repeat...do not make any decisions about this right now. PP's are correct. So much could change in their relationship a year from now, so don't even worry about it at this point.