There were four children in Hugh Fleming and Grace Stirling Morton's family, including Alexander Fleming.
Alexander was the third of those four children, born in 1881 at Lochfield farm in Ayrshire, Scotland. His father, Hugh Fleming, was a farmer, while his mother, Grace Stirling Morton, came from a neighbouring farming family. Hugh had also had children from an earlier marriage, which is why some biographies describe Alexander as one of eight siblings or half-siblings in total.
That larger number is the main source of confusion. Four refers specifically to the children of Hugh and Grace together; eight refers to the surviving children from both of Hugh's marriages. Alexander's full siblings included Grace, John and Robert. His half-siblings included Jane, Hugh, Thomas and Mary.
Alexander's family circumstances changed early: his father died when Alexander was seven. He later moved to London, where he lived with his older brother Thomas and continued his education before studying medicine.