AIO: Cousin has irrational hatred towards my gf

Me and my two cousins have been living together since October in a moderate trailer. Everyone has their own space except me and the younger cousin have to share a bathroom. I started dating my gf in December and she has been staying over here recently and I’ve been staying with her at her parents places. My cousin seems to have a completely irrational hatred towards my gf. There have been multiple instances where she has gotten mad or irritated at the thought of my gf or even being asked for one minute in the bathroom. I’m not sure if she thinks that my gf is below her in the way that she matters more and there for should not be “making sacrifices” by getting out of the bathroom for not even 5 minutes. The bathroom situation was the most recent one that happened not even an hour before me writing this post. But please help me understand if I am overreacting or if this is completely petty on my cousins behalf