Millie Pig is Molly Pig's little sister.
She has an invisible friend, Emma-Lou.
She is described by Ganz as follows: "Millie is Molly’s little sister. She loves worms and bugs, singing songs she makes up, giving her unsolicited opinions, telling long stories, and generally bugging her sister. She absolutely ADORES Roberta, a fact that occasionally bothers Molly."
Millie Pig can be found in the following books:
Millie's Sandbox (Unreleased)