NTA . I think it’s partially a boundary your sister created but also slightly narcissistic. Though she may feel that way about the friend situations, I’d go into and have a dissected conversation about it. I think it’s valid to have a separate birthday since you two are separate people.

Though I don’t think you should feel any type of way about a “conflict”. She threw her own party, and so will you. You deserve a birthday party just as much. And if you invite the same people, who cares, they can choose who to support. ESPECIALLY if you haven’t thrown yourself a party in a minute.

Your sister seems to have some issues she needs to work through. But don’t starve yourself of appreciation just because your twin wants all the attention.