Orphaned during the Dalek Invasion of Earth in the 22nd century, Ian was adopted by David and Susan after the human victory. He was named after Ian Chesterton, an early companion of Susan's grandfather, the Doctor.
Ian had two younger adopted siblings, Barbara Campbell and David Campbell Junior (PROSE: Legacy of the Daleks). Some time after the three orphans were adopted, David and Susan conceived a child whom they named Alex Campbell. Alex was, on some level, Ian's youngest brother. (AUDIO: An Earthly Child)