fight between adult and a minor

What is the name of your state? ohio

my 16 year old sister was involved in a scuffle with an 18 year old. what are my sisters rights, and do they change if she verbally started the brawl.

she has the right to get her ass kicked. Otherwise, based on your post, there's not enough information to form an opinion.

The fact that she's a minor doesn't mean a whole lot unless the 18 year old was her babysitter or otherwise responsible for her. If she gets into a fight with an 18-year-old, the whole thing is treated as any other fight.

Carl