Small spoiler but not really? Cause the artist draws lots of fanart of their own series haha.
If you look at art by the artist on their twit, they draw quite a bit of older art of the kids and its canon that shizuku, Minato, and aoi are their only kids. So no worries on that last part xD I was an only child for 12yrs before my half-siblings came around so it's weird for me to see an older sibling so chill with their younger half-siblings (was also raised by a single mother for a bit before being adopted by my dad at 5yo). But I had major only child syndrome. Also it's actually not unusual for people to have kids so close. A lot of the time people have the belief that they need to have two kids close together so their kid has a sibling to grow up with. I feel like Minato is also just going through what many middle children do.
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 ^^