Uncanonical sibling

The Uncanonical sibling is a type of Mary Sue.

She usually is the twin or younger sister of a canon character. Older sisters are extremely rare. Fandoms that are particularly plagued by uncanonical siblings are fandoms where the main characters are in the same age group as the author. Although there are also cases where the age difference between the siblings can be upto 20+ years.

If (one of) her parents is a canon character she is also an uncanonical daughter.