Which character is Mike Wheeler's older sister?

Mike Wheeler’s older sister is Nancy Wheeler.

Nancy is the oldest child of Karen and Ted Wheeler, making her older than Mike and her younger sister Holly. She is portrayed by Natalia Dyer and is introduced as a popular Hawkins teenager whose priorities initially revolve around school, dating, and her relationship with Steve Harrington.

Nancy’s story changes after her best friend Barbara Holland disappears. She joins Jonathan Byers in investigating the supernatural events surrounding Hawkins, eventually entering the Upside Down and fighting the Demogorgon. That investigation helps establish her as one of the show’s most determined and capable characters.

A common mix-up is confusing Nancy with Karen Wheeler, who is Mike’s mother, or Holly Wheeler, who is his younger sister. Erica Sinclair is also a younger sister, but she belongs to Lucas Sinclair’s family, not the Wheelers. Nancy’s family relationship is distinct from her later connection to Jonathan, which begins through their search for Will and Barb.