5 Beautiful Sisters

AU: Following Fuutarou's meeting with the quints and Rena's death, it was Isanari and his wife to adopt the 5 sisters. In this way the quints have Fuutarou as their brother that is always there for them, but they seem to feel something more special for him, and his parents even encourage him to marry them in the future. Which shenanigans will Fuutarou encounter in such situation?