I want to get in a yelling match with a guy who was trying to flirt with my sister
Was he trying to hurt your sister in any way?
No he is just a guy trying to flirt with her.
A guy was flirting with your sister and it made you furious, but you wanted to make a big scene out of it. That scene isn't needed, make sure you calm yourself down. Anger can make you say and do things you will regret later, arguing is so overrated.
You're right. I should mind my business.
But if someone is being too aggressive with your sister then you can definitely intervene. That would be the right thing to do.