Why is my 17 year old big sister always acting like an idiot, and annoying?
she acts like a 6 year old
She acts that way because of the stage that she is going through. She is at an age where her brain system that regulates logic and reasoning is not yet developed. The best thing you can do is to understand her and guide her where she goes wrong.
Because she is a teenager.
just let her be
Because she is in the stage of life where people are REALLY weird!!