Sounds decent enough for the PWP. New ones based on Prisma Illya: First, while everyone else in the family is out doing their own thing, Sella and Leysritt are home, drinking. Nothing major, just that they like to cut loose once in a while. Sella's a lightweight, so while Leysritt is barely buzzed, Sella is smashed. Then Shirou comes home, and, having seen this before, goes to help Leysritt carry Sella to bed. But unlike all those other times, Sella looks at him and can't restrain herself anymore, and sticks her tongue down his throat. Leysritt takes this to mean it's open season on Shirou, and readily joins in. The next morning, Shirou and Sella are both horribly embarrassed. Irisviel approves of the situation, and expects it to continue. Next: Illya, Kuro, and to a lesser extent Miyu all want their dear onii-chan. Mama-Iri notices this, and realizes that, if the much older and larger Shirou were to become a beast consumed by his perversion, he might wind up hurting them. Therefore, the only method to make sure nothing bad happens is to make sure Shirou is properly prepared to please his sexual partners, that the girls are properly prepared for him, and that they're properly supervised. Shirou gets dealt with in advance, since he needs the practice. He starts getting sent on "errands" where he winds up meeting up with Irisviel, who takes him and gives him detailed instruction and practical experience in every aspect of sex she can think of: oral, vaginal, anal, paizuri, and a bit of bondage, just in case. When she decides he's ready, he, Illya, Kuro, and Miyu find themselves trapped in what appears to be a love den with Iri, who explains that she is going to give the girls a demonstration of how sex goes, and then it's their turn. Miyu turns out to be a prodigy at this as well, though she wants Illya to be involved (she kept it a secret that she actually thought it was really exciting to have Illya watching her. Illya kept it a secret that she thought it was kind of exciting to watch her onii-chan and best friend together, with her being denied both of them. Kuro thought the whole thing was really hot, no matter which of the people there was involved). Kuro is delighted by the seal that makes her feel what Illya feels, since it effectively means twice as much sex for her. Plus, she's just all over anything lewd, so things like licking Shirou's cum out of Miyu while he fucks her from behind or her and Illya each sucking one of Iri's breasts while Shirou fingers them and Miyu rides his face are just what she wants. Illya is a little reluctant until she gets encouraged by her mother and the jealousy of knowing other people are fucking her onii-chan. Needless to say, they all end happily, and all the girls have an excess of prana. Kaleidostick and Class Card induced cosplay optional, but encouraged.
Third has spoilers for 3rei