Billy Joel isn’t the only member of his family with musical talents. British pianist and conductor Alexander Joel, Billy’s half brother, made his North American conducting debut on Friday at the Boston Lyric Opera’s opening night performance of “The Merry Widow,’’ which was written by Franz Lehar in 1905. Pictured: Alexander Joel (far left) with Boston Lyric Opera general and artistic director Esther Nelson and Alexander Lehar, grand-nephew of Franz Lehar.