Arriving at 9:21 a.m. and weighing in at 6 lbs., 9 oz., their daughter arrived on big sister Mia Loren's second birthday.
"We didn't plan it!" Ziering, 49, tells PEOPLE. "Mia was born a week late and Penna was a week early."
Mia was introduced to her new sibling last night, the dad of two says. "She couldn't wait to meet her. When she saw Penna she said, 'Baby sister!'"
As with Mia's moniker, the new baby's name draws inspiration from family. Penna is in honor of Ian's late father, Paul, while Mae makes her the fourth generation of women on Erin's side to use the middle name. Erin's mother, grandmother and great-grandmother all share it with the newborn.