Birthday invites

So my 18th is next month. I’m having a small thing in a hotel, I’m inviting my closest friends, my boyfriend and I’m planning on inviting his twin brother + girlfriend because I’m with them heaps. But we also hangout in a group with their other mates (like 4 more) but I will have too many people if I add the extra 6 people. I’m already pushing it by adding 2.

Would I be rude if I didn’t invite the extra 4? I feel like I would be and idk what to do!

My mum says I should invite his twin which I agree but idk!

We are only recently daring as well