Okay, so Takagi got married to Nishikata, they got kid, then divorced (or he died) and she is now with another guy. They are either not married, or that guy took her surname upon marriage (in Japan you have to change the surname to match surname of your spouse because of XIX century law which people try to cancel by suing the government in the court of law every year).
Judging by this chapter alone, it's a plausible possibility.