I guess it’s sometimes hard for Minato to be the child in the middle. I’m a single child so I don’t know but I think they had Aoi too soon after Minato. He can’t find his place and still want to be a mamma boy and hasn’t understand yet what it means to be a big brother. But we know he would be a wonderful big brother later.
But for now, it must be difficult for them. I think this is why Naoto fear a little his coming heat : no more child ^^