Olivia is an orphan girl who lives in an orphanage owned by Miss Grudge. She took care of The Chipettes while they were in Australia. She never found out why they left until they met her at one of their concerts after she was adopted in episode The Chipette Story.
Alvin and the Chipmunks (TV series)
Although the fact of how she ended up at the orphanage is unknown, (presumably due to the death of her parents and that she had no living blood relatives) she took The Chipettes in when they were just infants. While they became close as sisters, she kept them hidden from the rest of the orphans in fear that they'd use her friends' musical talent for their own benefits. Her concerns were valid, as Miss Grudge constantly makes the orphans practice singing in order to find the right musical group to exploit.
That is, until The Chipettes were discovered by Eva, who threatened to expose them if Olivia didn't give her her mother's old bracelet. This event had led to her friends to steal back the bracelet after seeing how sad she had become. But, in the process, they were caught by Miss Grudge and after she threatened them, she hid them inside one of the orphanage's vents.
When Olivia was finally adopted, she went into tears after Miss Grudge lied that The Chipettes ran away from her and that she would have to move on.
Years later, she would finally reunite with the girls at one of their concerts where they shared a sisterly hug after it was over. Soon they would tell their side of the story of what happened to them at the orphanage. Soon after the girls are done with their story, they, along with Olivia, go over to The Chipmunks' house for a party prepared for The Chipettes.