So the sister is in an unhappy marriage; she's been having an affair, it seems; and there are suggestions that she is pregnant with her affair partner's child. Meanwhile, she's also trying to come to terms with her brother not being "hers" anymore.
If I squint, I can see an incest route. But the tone of the story tells me the sister is headed a bad way of her own making. Her brother is going to cut her out of his life (if he didn't do so already) because of the sister's wariness towards his wife and his stepdaughter. The sister's mom will react poorly to the affair and likely disown her in some manner. The friend won't help her, because the sister is likely pregnant (again) in the worst context (whereas the friend can't get pregnant).
The sister will have no one except her affair partner, who will likely kick her to the curb when the bill arrives.
It feels like everyone will be miserable in this manga except the wife and the stepdaughter.