Hiring kith and kin is common in Hollywood, but Angela Lansbury has turned Murder, She Wrote (CBS, Sundays, 8 p.m.) into a veritable family reunion. The actress-executive producer has given jobs to her brother and her sons, but you won't hear CBS complain. After all, it's the familial atmosphere that keeps the 68-year-old star of one of the network's longest-running and highest-rated shows happy. ''I don't believe that I would have remained with the series if I hadn't had my family around,'' she says. Lansbury calls her husband, Peter Shaw, 75, an ''unofficial consultant'' for the show. Son Anthony Shaw, 42, became Mom's dialogue coach in 1984, in Murder's first season (''Nepotism reared its head early in the game for me,'' Lansbury admits), and will direct nine of this season's 22 episodes. Stepson David Shaw, 50, is president of Lansbury's Corymore Productions, which coproduces Murder with Universal and made Lansbury's 1992 TV movie, Mrs. 'Arris Goes to Paris. Her younger brother Bruce, 64, recently became a supervising producer for the series. Working in the family business has its ups and downs, but the clan insists their relationship is harmonious. ''We disagree, but Angie wins because she's the executive producer,'' says Bruce. Off set, titles are left behind. ''Even though Angie's working,'' says David, '' she still cooks the Christmas dinners.''