I believe the brother and the brother in law met first. They dated, but then they had the fight where the brother in law said the brother (MC) prioritized his sister, so to my understanding, that's when the brother in law went after the sister. I believe it might've been to break them apart like in chap 4.
It actually shows she pursued him knowing he likes her brother, so it's sort of unfair to say he "went after" her. She was a willing participant and had full knowledge to be fair lol
I do agree she was aware and informed. I mean he went after her because he also knew she was his sister and had no business even meeting her, let alone marrying her even if she insisted. It it to MY understanding (it hasn't really been suuuper confirmed) that he accepted (and even went along with her the first time they met) the marriage proposal with malice, and that his plan was to break them apart from the start.
Yeah he's definitely shady for ever even accepting it, even if the bf was ignoring him at the time! Everyone is a mess here lol
So was the brother in law dating her brother before they got together or has the brother in law always liked the brother