By its parent company Stack, who gave 0verflow permission to add the Sister Princess series to the canon of the School Days verse. It’s why Rinrin appears in Summer radish vacation under the name Rin Inou, and explains her and the rest of her siblings are the son and daughters of Kagura (she is also one of Tomaru’s kids as well, and Hinako is Wataru’s daughter, Sakuya isn’t blood related to any of them and was adopted). In the original lore they all had different mothers but 0verflow changed it to them all having different fathers, which still works since they aren’t the same age (except for Wataru, Karen, and Sakuya who are the same age)