Comments of chapter undefined of Sweet Doting Husband: Sorry, my Wife is a little Crazy

Something I don't understand is the focus of the minor thing. . He's the 3rd brother meaning that Lu Jin and Si Ming are still younger than him, so they're still minors and we never saw this topic being brought up... Is it only Mu Jun that cares? Are the other girls older? Or between minors is OK? Is not a big problem with the story just want to know why is it OK for some but not OK for others... Is it a personal trait of the character or just a gimmic of the story?