Do i need to invite my cousin's boyfriend who allegedly sucks?

If they are a couple and they are living together, you need to invite them together. I completely understand your reasoning for not wanting to, but that awkward situation isn’t yours to fix. That’s their family’s issue, and should be settled within their family. If her parents don’t want the boyfriend going with them, it is up to them to address that issue with their daughter. As the bride, you are only required to operate within wedding etiquette (ie, inviting couples together); you aren’t required to serve as judge, jury or family counselor. Don’t stress about it. Invite the boyfriend and let that family sort it out amongst themselves.

You really need to invite everyone that identifies as being in a relationship with their partner. It would be disrespectful to them not to.

I agree. Also, the parents and their adult children also don't have to stay together. If the parents choose to stay with someone they don't like, that is definitely not your problem to solve! If there are 100 guests or more, they can find ways to avoid each other.

Not inviting him would be incredibly rude. They live together not inviting him is disrespecting their relationship while asking them to celebrate yours. Best of luck to you

As the Host, it's your responsibility to invite guests recognizing couples as social units. It is not your responsibility to weaponize Your wedding for this family's feelings. It would be hurtful to your cousin and evokes personal disapproval at her life. But, like you say, you've never met him.

Any guest has the option to decline to attend, whether BF or the family members. If they can't act like adults at your event, then let them say so with their decline.