Is it wrong of me to tell my sister to get her shit together?
Why do you think you need to tell her that?
I think she's acting really childish
I think you might get a better result if you don't resort to swearing.  Cussing at a family member rarely leads to productive conversations.  Talk to her about why you think her behavior is problematic!
I'm her brother, what did you expect?
Cursing at your sister and putting her down may only lower her self-esteem. If you point out her potential instead, it may help her to start reaching for good opportunities.