Hildegard was singing about her bad first day at school when she was younger when she first mentioned her sister. When she was having a hard time making friends, she saw her big sister Astrid in the sun so she ran up to her and friends to be with them. The trio were doing their princess twirls and asked Hildegard to give it a try, but Hildegard didn't know how to twirl like them, so she fell down and Astrid's friends laughed at her. Astrid herself, however, did not laugh and seemed heartbroken at her little sister's humiliation.
Astrid makes a small cameo in "Forever Royal", where she sits next to her father at the ceremony where her sister and her friends graduate from Royal Prep.
She was later seen in the beginning of "Princess Adventure Club", chatting with Prince Hugo, and she looked exactly the same as she did when Hildegard first came to Royal Prep.
Her name hasn't been revealed on the show yet, but according to Craig Gerber, Astrid is her name.
Throughout the series, Astrid never had one line of dialogue.
Little Hildegard with Astrid and her friends
Astrid's friends laughing at Hildegard while Astrid herself is heartbroken