As members of the Duggar family, Jinger Vuolo, Joy-Anna Forsyth and their brother Joseph Duggar knew that once they wed, the next step would be a baby. “It’s pretty typical in the Duggar family,” Jinger, the sixth of Jim Bob and Michelle’s 19 kids, tells Us. “You just expect it.” So when the stars of TLC’s Counting On (Mondays, 9 P.M.) learned they were all expecting their first children in 2018, they couldn’t have been more thrilled. “Going through pregnancy with Joy and Jinger has been a really unique experience,” Kendra, 19 (expecting a son on Father’s Day with Joseph, 23), gushes. She and her sisters-in-law Jinger, 24 (expecting in July with Jeremy, 30), and Joy-Anna, 20 (who welcomed Gideon in February with Austin, 24), share their baby joy with…