Is it rude not to invite my sister's boyfriend?

I have to agree with OP on here....he is your sisters baby daddy so you have to invite him.

I don't like my dads wife because she talks shit behind my back to my family (don't wana go into full detail, but I'll give you a few instances...she loves to spread rumors that ain't true about me...even go as far as getting drunk on my daughters first birthday and forcing him to stay home to take care of her. Shes very jealous of me and our 2year old daughter for some reason).

I personally don't want to invite her, but I will anyway because shes my dads wife and I know he cares for her even tho she embarrasses him (he hasn't said it himself, but his sister-my aunt has told me this).

I'm hoping she wont come...but then again she might pull the same stunt she did for my daughters birthday-get drunk and make him stay home (shes a alcoholic and a pill popper btw)...that would piss me off.

Unfortunately you should. That's who your sister picked for her partner and the father of your niece. It could put your sister in a position where she feels like she has to choose. Invite and ignore him. Hopefully, you will barely notice he is there.

Invite him, and have a "bouncer." My photographer even asked me if there's any strained relationships in my family so she and my DOC can team up to avoid any problems.

Yes. You need to invite him. Great suggestions on having a bouncer or other person to look out and intervene.

He's her SO and father of her child that almost makes him family. Hopefully he'll behave.

I had/have the same dilemma. My sister's SO is a complete scum bag and really no one in the family likes him. I absolutely did not want nor do I want him at my wedding in June. He constantly says nasty things about myself and my family. I can't stand him to put it nicely. However, I realized that I had no choice but to invite him. I hate him, but my sister loves him. It's hard to accept, but you can't choose your sister's SO. I cringe thinking about him being at my wedding, but it's my day and I'm not going to let his presence ruin my day if he chooses to come to the wedding.

One thing to consider is; he or anyone else you invite will be in your wedding pictures or video forever. We've had to make changes to wedding albums because sometimes people break up.

Wish you all the best.