"Isaac, in turn, is related to someone called Gerald. Gerald was the son of Ignatius and Lucretia Prewett, and family relationships both with the Weasley family and with Black, respectively. Gerald had a much better life than that of Marius, and apparently turned professional as a stockbroker."
Gerald Prewett was a squib son of Ignatius and Lucretia Prewett. He became a stockbroker or an accountant in the Muggle world. This profession was enough to embarrass the whole Weasley family.
He had previously been rude to Mr. and Mrs. Weasley, he and his wife were to have sought the Weasley family's help in introducing Mafalda to wizarding culture when they learned that she would be attending Hogwarts
According to Isaac Prewett, Molly Weasley had already visited once or twice with her husband, but that was only as Gerald was still alive. They were not very close so it was rare for a visit the other. He got very happy when I found out that Isaac was born with magical abilities.