Is OTHER wrong in the post ? POST : Throwaway in case anyone involved is on Reddit

I'm graduating uni in a few months, and my brother is flying in from out of town to see me graduate, and is brining his girlfriend. My dad, sister, and boyfriend also plan on attending, but since we only get 4 tickets, the first three we're going to go to my family and the third to my boyfriend. 

When my sister found this out she blew up at me, insulted me, and insisted my brothers girlfriend get a ticket over my boyfriend, since she's flying in. I like his girlfriend, but I've only met her twice and my boyfriend has been supporting me ever step of the way for the last two years of my degree and I don't think it's fair that someone who I've met twice gets to go over him. Also, she's not coming solely for my graduation. They'll be visiting other members of our family, then going on a cruise that leaves from the port the next city over, which they were planning on going on before I even had the date of graduation.

Would I be being rude or an asshole for having my boyfriend go instead of my brothers girlfriend?