Responses
Because he couldn't find her at all, judging for the ex duchess to avoid the church for so long, it isn't surprising she avoided the Duke even better, plus Duke didn't want his son to have the label of an abandoned child so he needed to find her fast or said ex duchess is dead.
FL and brother were adopted when FL was 7 so I think Duke remarried a lot later, at least judging with the fact that white haired brother (forgot names) is older that FL and it shows he was pretty small when duchess disappear
Chapter 84 was so sad and to think the ex duchess was still alive along with Rania until 3 years ago makes me wonder why he remarried instead of trying to find them. That too he remarried soon after since both brother and sister grew up in the household along with their step brother