Does it sound like I am inviting myself to my cousin's party?

I would simply out it to your cousin this way: "hey, I would really love to come to your grad party, but if you had a party with just your friends in mind that's fine too". If she says "No, I want you there" then you know she simply forget to mention it. If she says "Actually, Yeah, I kinda do just want to have it with my friends" then suggest you, her, and your other cousins your close to can celebrate some other time. Don't feel offended either way though, it's possible she felt uncomfortable about introducing her friends, or felt worried about unintentionally leaving someone out. Best of luck!