Looks like a lot of people is misunderstanding lil bro's age because he is so short, but he is a first year high schooler (15-16yo in Japan), so in another 2 years and few months he is gonna be safe game for our ara-ara-onee-san. And dunno how old do you think Natsuki is, but she should be in her early-mid twenties, so when he is 18 there shouldnt be that big of an age difference to make it awkward.