wedding plus one

We have been dating 3 years and are 20. He lives with his family so he get the invite with his parents. It's his cousin who I met and know she give plus ones to other cousins (people who aren't in relationships or are no matter how long they have been dating). My parents think it's odd I'm not invited as his date since they know me but invited people they don't know. We will be living together next year will that change future weddings? I saw some people on forums assume he doesn't need a date since he's with his family but his older cousins will have siblings and parents there and they get a plus one? What are your opinions? I know I'm not invited just wondering others opinions. If I was her I would have invited couples like this especially if I knew them and invited my other cousins plus ones. Just because we are younger adults seems like we are treating like kids/not a serious relationship. I wonder what my bfs moms opinion is but wouldn't dare ask lol. We are under 21 so does that make it an issue???