Doesn’t it seem odd that the Weasleys are more or less the only siblings at Hogwarts? That’s not entirely true, of course – there are minor characters with siblings, like the Patil twins and the Creevey brothers – but the vast majority of Harry’s classmates are only children. Hermione most likely is; in Deathly Hallows, she modifies her parents’ memories in order to protect them, but doesn’t mention doing the same for any siblings. Draco, Neville, and Luna are all certainly only children, and if other supporting characters like Cho Chang or Seamus Finnegan have siblings, we never see or hear of them.
There’s probably a very good reason for that: although the Harry Potter books are long and full of detail, they are well put together. All those details usually end up being relevant in some way, and if it’s not important for Hannah Abbot to have an older brother, she probably won’t have one.
So why are the Weasleys such a remarkably big family? I think the answer to that is in their role in Harry’s life. Harry comes from a family in which his cousin Dudley got two bedrooms while he slept in the cupboard under the stairs, and he grew up getting hand-me-down socks for Christmas while Dudley threw a tantrum if he got 36 birthday presents instead of 37. In other words, he was not treated like a part of the family at all. The Weasleys, on the other hand, have seven children and love all of them. They welcome Harry into their family and treat him like their own son in a way the Dursleys never did. In a way, they are the anti-Dursleys, a loving family Harry chooses to become a part of to contrast with the blood relatives who barely tolerate his existence.