Nao adores her big brother. This adoration usually takes the form of sexual torment: teasing him with glimpses then snatching them away. When Nao discovers she’s adopted, she only becomes more determined to snag him, or at least dominate him; but she faces some competition from a girl who knew him (almost biblically) when they played “doctor” as children, and who’s now developed an obsession with him. They’re eventually joined by a girl with a taste in boys-love manga, who somehow thinks it’s better if he’s the one seen buying it. The battle between the non-blood-related-sister, the stalker and the yaoi fangirl, supposedly over his affection, regularly devolves into seeing who can torment him the most.
Arriving shortly on the heels of the previous series, I assume this was made to cash in on it. Where Ore no imouto was at least a little bit subtle, Onii-chan no koto… (which shortens to Ochinko, not a word to use in polite society) is utterly blatant. Panties? Ever-present. Cleavage? Not really, but flaunted anyway. Perversions? Galore. Masturbation jokes? Aplenty.