Harriet Nelson, once a TV mother, is now a TV sister. The motherly half of Ozzie and Harriet will appear on NBC's "Father Dowling Mysteries" Friday night with her granddaughter, Tracy Nelson, daughter of the late Ricky Nelson, and with Tracy's husband, William R. Moses. The last time Harriet and Tracy worked together was some 25 years ago when all the Nelson grandchildren were on an "Ozzie and Harriet" Christmas show.

"It was fun," Harriet said of the "Father Dowling" episode, in which she plays a feisty nun known for her potent rum cake. "I'd always wanted to play a sister and I have a great regard for Tracy as an actress. I'd forgotten what fun it was to work again." Harriet's last full-time job was for a television pilot that didn't make it two years ago but she could pop up on "Father Dowling" again sometime. "The executive producer, Bob Hamilton, is great," she says. "He said, `Any time you want to work, just pick up the phone and you'll be on the next show."'