As far as I can tell (I might have missed some subtle hints), there really isn't much about how/why that happened. The only direct allusion I'd see would be in that scene between Konatsu and Yakumo, right before his death.
The official subtitles:
Yakumo: What's all this? You need so much coddling, today...
Konatsu: I'm a slave to my blood.
Yakumo: You've made up your mind, then?
Konatsu: Yes. But I can't give a name to this feeling anymore.
Now, I really don't want this to sound like I'm taking a dig at the translation, as I think it was cleverly and often beautifully done overall, but I disagree with the "you've made up your mind, then?" line. The way I understand "ふんぎりはつけたってエたろ" is more "I thought you said you had gotten that out of your system" (... except that's definitely not a very Yakumo-like way to put it, sorry... English isn't my first language, so please bear with me), which really seems (in hindsight, anyway) to hint at a conversation they had around the time of Shinnosuke's conception.
The "slave to my blood" part (literally, "my blood is making me do this") also brings to mind the conversation Konatsu had with Yotarō, earlier in the series, when she was worried about "her blood making her do things" and possibly becoming like her mother. And just before that, there was another scene where Oei said Konatsu reminded her of Miyokichi, Konatsu was offended, and Oei then said "oh right, you're completely unlike her", which... might actually have been sarcasm, in hindsight (based on Konatsu's comments to Higuchi at the end of the series, Oei and Kido (the yakuza boss) knew who Shinnosuke's real father was).
