Harry and Ginny already have three kids, Lily Luna Potter who is 1, Albus Severus Potter who's 2, and James Sirius Potter who's 3. But now there is a forth, Tyler John Potter (after Remus John Lupin) also known as TJ. Lily thought have a baby in the house would be cool, but then she came to realize that babies could be a pain. James and Albus already knowing what it's like to have a baby in the house, weren't as excited as Lily. They were happy to get a brother, but not so happy when the time came for him to come home. The boy were expecting a very long talk about responsibility like they had with Lily but instead they just got a do's and dont's list. Like don't throw the baby around, do feed the baby (with mommy or daddy's help), don't draw on the baby, don't give the baby any non-baby food. But the boys actually were the ones who helped out a lot, while Lily was the one who was more or less inefficient with the baby. But all that brotherly bonding paid off as Tyler (TJ) got closer and closer to the year he joins Hogwarts.
I hope you enjoyed that little flashback before we dive into the deep pieces when the Boys and Lily are older.
