Yeah I was thinking the same thing, she was cheating on the husband with Miki and that's why he pulled off his nails.
You people are so smart that’s why he said whose son was this
Mabye the kid looks like him simply because it's his nephew and on the night of his assault he was only trying to stop the husband from raping his sister... Or he was in fact the woman's lover and his plan was to get Tatsumi to kill the yakuza and then run away with his lady friend but he eventually realized that, maybe, he loved Tatsumi more than her...
I like the fact that we'll never know.
i have a feeling that the woman wasnt actually miki's sister. he was probably shacking up with her and her yakuza husband found out. considering his last words were that everything he said were lies, im pretty sure that goes for her not being his sister.