Squeak

Squeak was the nickname of Midge's younger sister. After Midge returned to Earth from the Cheetah World, now infected with the Cheetah Virus and under the Master's control, she saw her mother being transported to the Cheetah world and her cat being killed by the "bad cat" the Master brought. The Seventh Doctor and Ace later arrived and found her crying. After learning what had happened, Ace took her to her nan's flat upstairs. (DW: Survival)

Midge's sister died in the early 21st century when her immune system gave out, having been weakened by severe pollution. (NA: Cat's Cradle: Warhead)