Cuddled by a doting big sister who spoils you with whispers and pussy... A naughty big sister who's cared for you since you were both young. A luxurious life enveloped by warm pussy and big tits.
The setting is a "delivery health" (dispatch brothel) service where all the girls are from "Isekai" (other worlds), and it would seem that semen is the only source for replenishing their mana in this world.