Wtf ? 7 ? Maybe 8 ? Bro that's just having stupid parents if they aint rich learn to use condoms or do a vasectomy/womans equivalent, dont be a hero, Japans birth rate is on the decline sure but this is not the answer.
They just dont give a fuck about the problems the kids will face, hell as its seen from here they never got enough attention from the parents and end up depending on the older sister, you gotta spend a ton of time with them as well this is just sad to see.