The royal family is counting down the days until they meet their newest member! According to an insider, there is one particular person in the brood who is especially ecstatic that Prince Beatrice and husband Edoardo Mapelli Mozzi are expecting their second child: the couple’s daughter, Sienna, 3. “They are so excited to be growing their family, but Sienna is probably the most thrilled,” the insider said of the family-of-four, which also includes Edoardo’s son, Wolfie, 8, whom he shares with ex-fiancée Dara Huang. “She wants a little sister to dote on. Beatrice has had to remind her that she c…