Headcanon: Before Narnia, Peter and Edmund would fight a lot. Though Edmund was often in the wrong, Peter would make some biting remarks. Susan would always get annoyed with Peter since the latter never followed Edmund when he ran off. He never found him, like Susan, in his room, clutching a photo of their father to his chest, and sobbing for him to come home. Susan would then wrap her arms around her little brother until he stopped crying and usually fell asleep.
(Gifs from roleplaygateway and piclist)