How to obtain guardianship of my 18 yr old sister?

My 18 year old sister currently lives with our father in Florida. Right now she is about to finish her junior year of highschool, which leaves her with 1 year left of school. My sister would like to move in with me & my husband in North Carolina because she doesnt have a healthy relationship with our father. However, she cant withdraw herself from school because she must have her parent/legal guardian to withdraw her even though she is 18 & legally considered an adult. Unfortunately my father doesnt want my sister to move, he wants her to live with him til she's in her mid twenties at the very least. I really dont want to go through the hassle of taking my father to court to get legal custody of her. My question is: Can I become my sister's legal guardian even though she is 18 & how to?