"Tarlie" is the relationship between the Duncan siblings Charlie and Toby Duncan.
Toby is Charlie's unexpected younger sibling. It was revealed that Amy was pregnant again and that Charlie would have a younger brother or sister in Good Luck Charlie, It's Christmas! the latest Christmas/Holiday Movie. It is inferred that Charlie may have a sense of jealousy towards the new baby, due to her no longer being the baby in the Duncan family. Later, she asks Teddy if Bob and Amy "take the baby back (from the hospital)".
This was unjust, despite the fact Toby was born on her third birthday. Charlie said that she liked Toby as "her present".
However Charlie was shown to still hate Toby, such as when she tried to mail him away in Team Mom, Or put all of his toys in the driveway and throw away a doll that Amy gave her in Guys and Dolls. But she grew to love him after Amy played a trick on her by pretending Toby could talk and told Charlie not to tell anyone his secret. However it is shown that Charlie still doesn't like Toby, as in the end credits of Charlie 4, Toby 1 when she wished Toby was gone after blowing the candles on her cake and making a wish. During the end credits of Futuredrama Charlie asks her fourteen year-old self if Toby ever moves in with her Grandmother. The elder Charlie sighs and says she's still working on it.