I suppose that you would say that Olivia falls in love with Cesario instead of falling in love with the Duke of Ilyria, Orsino. The reason I would say this is that the Duke was in love with Olivia. He sent Cesario, who is really Viola in disguise, to woo Olivia for him. But Olivia was convinced that Cesario really was a man and so she fell in love with "him." She simply says it is because of Cesario's "perfections."
Olivia ends up marrying Sebastian, who she thinks is Cesario. Sebastian and Cesario look alike because Sebastian is Viola's brother.