I'm mad at my cousin for how they are planning my wedding.
You're mad that they are helping you?
Yeah, they are ruining everything! It's awful!
If your cousin is planning things incorrectly, tell them! If they're making mistakes in the process, correct them, but be grateful you have help.
I don't want their help!
If you don't want their help, then you should calmly explain that to them instead of yelling at them. Be sure to tell them that you do appreciate them being willing to help though so they feel like you are grateful for them. If you calmly talk to them, they will understand where you're coming from.