After re-reading it and looking at the hand moves and the dialogues, I think there's more than meet the eye here. It's not only a cheating story.
What I understand is that it's a tug war between the sister and the girlfriend. The girlfriend is dating the brother so that the sister "gives up" on her brocon (page 4).
So yeah, it's toxic, but it looks like the girlfriend was waiting for the sister to come home and uses the brother to show her that her bro doesn't care about her so that she moves on from her brocon.
And the sister wants her bro to dump her, both because she thinks the gf is "trash" (and she loves her bro) and because she wants her for herself.
Beautiful.