David Foster and daughters Erin, Jordan and Sara have a new tradition: family Sundays. Each week since the coronavirus lockdown began, they have congregated in the backyard of Erin’s Los Angeles home to talk about life and politics and share a glass of wine—and plenty of laughs. “Her backyard has become our playground,” says the hit record producer, 70, also dad to Amy, 48, and Allison, 50, from previous relationships, who live in Oregon and Canada, respectively. Though the family is now close, they’ve had a painful past. In 1986, when Sara, 39, Erin, 37, and Jordan, 34, were kids, David divorced their mother, his second wife, Rebecca Dyer. “I missed a lot because I didn’t raise them,” says David, now married for the fifth time, to singer-actress Katharine McPhee,…