My sister is breaking up, but I'm not going to take her side.
But she's your sister. Why would you choose her ex over her?
Because my sister is wrong in this situation and I'm tired of listening to her arguments.
I'm sure you're frustrated with your sister, but it's important to listen to her point of view and support her. Listen to the full story before supporting one side over the other and then make your decision. You may feel differently after giving the situation more thought.
I doubt it. I hate my sister so much.
If you don't get along with her, it's understandable that you wouldn't want to support her. Still, that doesn't mean you need to pick her ex's side. You should focus on who's wrong, not who you dislike more.