Charlotte Benton

John Benton's sister was the younger sister of John and Chris.

John called her his kid sister. John often met up with her. During his days with UNIT he was going to meet her at eight o'clock outside the Chinese takeaway. He was planning to take her to a dance at the Palais. He phoned her and told her not to be late. (TV: The Android Invasion)

Behind the scenes
John Benton's sister wasn't mentioned in HOMEVID: Wartime. Since John called her his kid sister she might not have been born in 1944. It's also not mentioned whether John Benton's father is her father or not. Since Benton's father died very early and Benton's sister is much younger than Benton she might have been the child of a new partner of Benton's mother.