Help! Do we invite toxic bf?

That small a wedding, it is a family event, not a mostly couples event. And he is hostile to multiple family members. This is not merely that you dislike someone harmless. Talk to sister, say you have nothing to say about her relationship, that is her business. But after his recent bad behavior, toward family, he is not welcome at this particular event.

I think In such a small wedding and given the situation it is fine not to invite him

If you don’t invite him you’ll probably be wrecking any relationship you may have with him in the future but if he’s that bad I assume you don’t care about that lol, so don’t invite him. What would his sister feel about not inviting her boyfriend...?

I would definitely talk to his sister and explain. Not inviting him would probably wreck your relationship with her as well unless she's onboard with the why, and even then its touchy because he'll probably somehow spin it. Just remember you're marrying into his family, so as much as it sucks...treat her like your sister. Would you deny a sister bringing her partner, whom she plans to marry, to your wedding? You may be able to set ground rules - aka "John must stay away from FMIL even at our small wedding and has to behave" - so when he inevitably messes up you can be like "All I asked for was for John to not cause drama today". Good luck! It's a tricky spot to be in, but if she plans on marrying him then not inviting him will probably cause some friction.

Are you planning on not inviting his sister? You think they may get married before you. If they do, that will be his sister's spouse. You don't have to invite any of his family, obviously, but I don't see how you can invite her without her husband. If they're still engaged? That's a different story. I would speak with her before invitations go out to explain why he's not invited.

How does she feel about how her bf and his family treat her family?

I agree with pp's, although there has been some bad boyfriend behavior - and that this guy may be a part of the family before your wedding day - that you should extend him an invitation. Although he has not put his best foot forward with your in laws, singling him out as the only one not invited will not only hurt your + FH's relationship with your FBIL, but also likely cause tensions among the entire family as I have a feeling FBIL and SIL will be fighting non-stop to convince everyone he should attend.

This doesn't mean he gets a free pass to behave disruptively or rudely at your wedding - in advance, FH should have a conversation with his sister about the behavior before formal invites go out (so still plenty of time to see if he cleans up his act on his own) and let her know that, based on what he has witnessed, he is worried about how FBIL treats FMIL and possibly SIL as well. She'll likely get defensive, and that's okay! But she should also be aware in no uncertain terms that bad behavior will not be tolerated at the wedding and he would be asked to leave if he causes a fight with FMIL. Best case scenario: he realizes the error of his ways and starts mending the relationships with your future in laws and you + FH well in advance of your May wedding. Otherwise, let FH talk with his sister, and make an effort to put FBIL and SIL far apart from FMIL during the ceremony. Good luck!!

I would actually ask your FH's mom before anything. If he's made her cry, she may not feel comfortable with him there. If she says she'd be happier without him, then have your FH and/or you discuss with the sister as to why you won't be inviting him.

Thank you all! We’re going to talk to his sister and go from there. Fingers crossed!