What is a female bro called?

If you are addressing a brother you could say bro and for a sister you could say sis, but I personally wouldnt say sis to a girl who was not my sister unless she was exactly like a sister to me (a close lifelong friend that was considered family).

Can I say bro to a girl?

Debatably, bro is the most common gendered term of casual address used by women for women. Until the 20th Century, the word was merely an abbreviation of the word brother, implying a male sibling, a religious title in Church, or a way for African-American men to express solidarity with each other.