Hi Johnny,
In this case "bro" is slang, like saying "buddy" or friend. It can be used to refer to any man, but generally calling a woman "sis" is used only if she is actually your sister. The greeting between girls can be different depending on their age. I'm thirty and might address another woman as a chick, or in a group, ladies. My teenage students with refer to their friends as "chickas", or "bay".