When a girl calls a guy "bro," it typically means that she sees him as a close friend, and the term is used as a friendly, casual, and often affectionate way of addressing him. It's similar to calling someone "buddy" or "friend." In this context, "bro" is short for "brother," implying a strong bond and camaraderie, akin to siblings.
Using "bro" doesn't necessarily mean that the girl sees the guy only as a sibling; it's more about the level of comfort and closeness in their relationship. It's often used among friends, especially in casual or informal settings.
However, it's essential to consider the context and the specific dynamics of their relationship. If the girl and the guy are dating or have romantic feelings for each other, calling each other "bro" might not be appropriate or might indicate a platonic stance rather than a romantic one. In such cases, it's best to communicate openly about feelings and intentions to avoid misunderstandings.