The post is more complex than the title, so let me elaborate. I've been dating my GF for almost 5 months now. A big topic for discussion is what she is going to do for her 21st birthday which is upcoming. She has a twin sister, which obviously has the same birthday, but she goes to a different college, one known for their 21st birthday traditions more than the university we go to. She has been wanted to celebrate it with her sister, but every other week or month she would get in phone fights with her sister and threaten to not spend it with her. Last Halloween they got into a big fight as she went there with a friend and her sister felt as though she was hanging out more with her friend than her (Sorry, it's maybe not the most relevant but some people might ask for it).

So whatever, as of a month ago she was definitely not going to spend her birthday with her sister. I was talking to her around Valentines day about what she was going to do now, and she said that she booked a bus ticket back to her parents for spring break the day before her birthday and was going to a concert with her mom. I was a little upset but it's her mother and I couldn't really invite myself to something like that so I said ok, I'll do something else for your birthday. This event got canceled and her and I were excited to spend her birthday together cause she could move the bus ticket to another day. I was asking her what we were going to do now that she has her birthday free and I was giving suggestions, go to a bar and chill with some mutuals, go clubbing, chill at the house, hang with her friends, or drive to her sisters school and celebrate with her sister (or do the 21st traditions without her sister). She states that she liked the idea of surprising her twin for her birthday, but I haven't cleared out my schedule yet because I didn't know where she would stand on that for 3 weeks.

Turns out, she had an argument on a phone call with her sister later that day and she told me we were not going to go to her sister's school for sure. OK cool, still no plans have been made and we are back at square one. A few days pass and she calls me telling me that she had a phone call with her sister and she bought a bus ticket to her sister's school after I offered to drive her there. She never stated if I was invited but it felt as though I was excluded from plans I've been trying to make for her. She then said that it was because I was "wishy washy" about my own plans and she needed a concrete way of getting to her sister. So I ask her if I'm invited and she pauses and basically tells me no. It feels like a gut punch, to basically have to beg your gf for an invite to her birthday. She's like I'm not inviting any of my friends, not just you, it's going to be me, my sister, (and definitely all of her sister's friends). It feels like she took plans I was trying to make with/for her and excluded me. I get that it's her birthday, but I would want her at my party. I also get that it sounds childish to be upset about an invite, but I feel like it more than that, like I am an after thought.

I don't know, everyone I talk to is on my side, but obv some personal bias.

What should I do, I'm definitely not going to drive separate from her to meet her for her birthday and roll over like I feel like I have been for most other fights.

I probably left out some important details so if you need them I'll edit.

Edit:
It's a 21st birthday, so can't really do 21st things before hand
Next day she is leaving for a week and a half