The sister is delusional and egoistic that can't blame his own ex-husband and instead blame her brother(mc) that only knew of their(sister & brother-in-law) relationship when it's too late. The ml is stupid at making decisions and doesn't clean his own mess.
Overall, I pity the mc for having a sister and a lover like them.
...though, it's still a good story with proper execution, unique characters, and original storyline.
Brother In Law