Just before Christmas, Tom Riley, 80, and his daughters, Kate, 51, and Erin, 48, did what many families do around the holidays: They exchanged greetings with their cousins over Zoom. Gathered at Erin’s Albuquerque home, they saw the squares on the computer screen filled by Gail DeCosta, 75, in Charleston, S.C.; Vicki Davis Williams, 73, also in Charleston; and Williams’s sister Julia-Ellen Craft Davis, 74, who logged in from her daughter’s home in New Jersey.