They are ramping up his desire to have kids. Him hearing that his sister is pregnant is making him jealous, and making him suffer. He is also slowly getting used to the Toukas advances. They are basically boiling the frog (dad)
Man, you can replace Yutori with just pair of boobs. It's been 49 chapters and there was 0 character development or involvement from her. It's like editors wanted big boobs highschool girl, but author couldn't find a place for her. So now she is just here for fansevice.