Dad’s girlfriend’s son

So my dad and I aren’t too close and I don’t see or talk to him often. He has been dating his gf for like 2 years or a little more and they live together. She also has a son that’s like 19 or 20 maybe that also lives with them.. I have never had a conversation with him, ever. I’ve seen him when I’ve gone over there but he hasn’t even hung out while I’m there. I feel like I have no passion for him to be at my wedding. My mom says to rethink that because it’s his live-in girlfriends so. But like I said, I’ve not talked to him once and he’s not someone I need there to see my get married.

I’m also going through this with my moms brother, he’s been dating this woman for a long time, and she has a son that I think lives with them as well, probably around the same age as the other, but I have not ever had a conversation with him either. My mom thinks I should invite him too and I don’t feel that way.

What do I do? Am I justified by not inviting them? Am I selfish by saying this is my wedding, I’m not inviting someone I don’t want to? I have friends I would like to invite but am probably not going to because it’s expensive so why would I invite two people plus their guests that I’ve never even talked to. My fiancés family is huge so that took up a lot of space for us.