In Norway, the rules of succession are pretty strict. To be a full member of the royal family, you need to be born into it or marry into it. Since Marius was born out of wedlock, he doesn't quite fit the bill. It's like trying to join a private club without the right membership card – you can still hang out, but you're not a full member.
This doesn't mean Marius is ignored by the royal family, though. He's still part of the family and attends some official events. It's just that his role is more limited compared to his younger half-siblings, Princess Ingrid Alexandra and Prince Sverre Magnus. Imagine being the cousin at a family reunion – you're still family, but you're not the center of attention.
