Оттуда же, откуда известно про 2-х братьев Артура.

Some Random Facts About The Weasley Family

Ron was the only one of three major characters whose surname never changed; he has been 'Weasley' from start to finish. In Britain and Ireland the weasel has a bad reputation as an unfortunate, even malevolent, animal. However, since childhood I have had a great fondness for the family mustelidae; not so much malignant as maligned, in my opinion.

There are also many superstitions associated with redheaded people and most state that they are in some way unlucky (Judas Escariot was supposedly red-haired), but this is nonsense; I happen to like red hair as well as weasels.
Although I never meant him to be like Sean, once I got Ron onto the page he often behaved like my oldest friend, who is both very funny and deeply loyal. However, there are also substantial differences between Ron and Sean. I have only once set out to faithfully depict a real human being (see Gilderoy Lockhart); everywhere else, though I might have borrowed the occasional real person's characteristic, they are at least 90% imaginary.
Before her marriage Mrs. Weasley was Molly Prewett. As you will note from chapter one, Philosopher's Stone, she has lost close family members to Voldemort.

Arthur Weasley was one of three brothers. Ginny (full name Ginevra, not Virginia), is the first girl to be born into the Weasley clan for several generations.

Fred and George were born - when else? - on April Fool's Day.