Susan’s best friends with Rosie (and is also fairly close to Charles) and one of her fav things she likes to do occasionally is take Sophie out shopping or to the beach or to a park to walk around or something, she considers Sophie to be the little sister she always wanted
It’s also cuz of Susan that Sophie was able to properly realize and come to terms with being no, she had decent guidance and healthy advice on the subject during her early teen years thanks to her