Sarah, informally known as Sarah-Lou, is the only daughter of Brian Tilsley and Gail Tilsley. Questions were raised on whether or not Brian was Sarah's real father and was quite a scandal at the time and when he found out Gail was having an affair with his cousin he told her to have an abortion, which she refused point blank, and for a time baby Sarah was rejected by Brian and until he found out he was in fact her real father he kept his distance. He was murdered just two weeks after her second birthday, and she was adopted by her beloved stepfather Martin. Sarah always acknowledged Martin as her father, even though she always called him by his first name.
Taken from Wiki
http://en.wikipedia.org/wiki/Sarah_Platt