I don't know how to react anymore. My sister and I both think we are total assholes. I'll do something mildly rude and she completely overreacts in my view. She says I'm not mildly rude I'm the biggest asshole on the planet and deserve no happyness
Maybe you should try to sit down and have a talk with your sister and get this worked out. If you're both being rude to each other you are both to blame.
She's not being rational. My actions are nowhere as rude as she presents them to be
Sounds like you two should talk this out with a mediator. A mediator can get down to the bottom of who is being rude or not.
All I know is she needs to grow up
If you are unwilling to see things from her point of view, maybe you need to grow up. She thinks you are being rude and it would be helpful if you saw it from her just once.