Sis is the all around language for "Girl." It could also be short for sister. However, when you use sis, it could range from high to low with the meaning behind it. In this case, this is how we refer to our homegirls when we see them, or when responding to a question that has far more beyond just a 'yes' or a 'no'.
Sis, she has no idea
Sis, lemme call you back ! He's about to walk in.
Nah sis, He's for EVERYBODY
Sis! it's wayyyyy more to the story!
Sis, It's all love, but that ain't it!