Presumably the implication is that glasses girl is mad that hot girl is dating her brother instead of dating her.
But also she gets called a bro con so... Not necessarily the case.
I feel like she's just teasing her, since the sister always uses the brother as an excuse to get closer to the girlfriend. But I guess it's not 100% clear.
Yeah I'm at least pretty certain that it's teasing. The girlfriend is saying that the sister talks about her brother all the time, and the girlfriend is poking fun at the sister for that. The story however is entirely focused focused on the girlfriend and the sister, the brother is not actually physically present in it, and all of the sexual tension is between the girlfriend and sister. So I'm confident that the girlfriend and sister is the author's intended focus, and the point of this short story.