[Shinobu Tanei] Imouto no Otsukai (COMIC Momohime DEEPEST Vol. 003) [ibndgfan]

A meek crossdressing stepbrother gets summoned to run his stepsister's errand, only to learn the bags she can't keep at home are full of sex toys that are meant to be worn, not delivered. Locked in a chastity belt with toys buzzing away, he's made to parade through crowded streets in a schoolgirl uniform until his knees buckle beneath a park streetlight after dark. Collared, gagged, and put on all fours in the grass like a pet, he can only protest that he's 'obviously her brother' while his smirking sister straps in and claims his ass as her own.