The battle of Hogwarts /D.M.
What if Harry Potter had a sister? A twin, precisely. What if Lily had protected two children that night, not one? What if a piece of Voldemort's soul, as well as ending up in Harry, ended up in his sister? Harry's life at the time he lived with the Dursley family would have been a little more comfortable, with his sister with him.
Dudley would have bothered him less and returning to Privet drive every summer would have been more fun, especially with Dudley starting to be afraid of the two wizards, especially Esmeralda, who had always been tougher than her brother. And so it was. Harry in addition to Hermione and the Weasley family also had his sister, and life was lighter.
