Hm, hm. This is a situation I thought up myself, but I forgot all about it. When you consider how easily they die and the number of children that have to be raised all at once, they've probably reached their limit. It would be different if they could raise themselves the way fish and insects do.
By the way, in this picture, the big sister Reimu taught her little sisters to do rub-rub, and they [ended up] refreshing and all dying.