Iman Zainab Javed was killed when a lorry collided with two cars on the M61 near Bamber Bridge, Lancashire, at about 22:30 GMT on 23 January.
Her family said she had had a "special bond" with her cousin, who she helped out of the wreckage before she died.
Police said the 11-year-old's father, who was driving, remains "critical".
Iman, of Rochdale, was in the back of a VW Sharan heading southbound that collided with a Renault Clio before a lorry struck both cars.
Three other people suffered less serious injuries in the crash.
Her family said she was " beautiful, caring, fun-loving, inquisitive and sometimes cheeky young girl", who "could play patiently for hours with her younger cousins but could also tell you the impact Brexit had on the pound".
"Iman had a particularly special bond with her three-year-old cousin, Aaima Vasim, who suffers from a serious heart condition," they said.
"Iman's last act in this world was to help Aaima out of the vehicle to safety."
Linda Cotton, Iman's head teacher at Norden Community Primary School, said her death had been a "tremendous shock".
"Iman was a first class student and such a bright, intelligent girl," she said.
A young girl who died in a motorway crash used her "last act in this world" to help save the life of her three-year-old cousin, her family have said.