Harry was the favorite kid and he wasn’t even an official part of the family
Because Molly knows exactly how the Dursleys treat him. There’s no way Ron wouldn’t tell her. And Molly Weasley is a Mother. She gets a capital M because she is goddamn phenomenal at what she does. When she hears Harry Potter is on the train to Hogwarts in Book 1, her reaction isn’t to be starstruck. It’s to say “that poor dear had to come here all on his own.” Molly Weasley loves harder than anyone. She loves like it’s her sole reason for being. And when she hears there’s a poor boy who has never known love his whole life… how could she not?
In Year One Molly Weasley knit Harry a Weasley family sweater and made him homemade chocolate so he would have something to open on Christmas DON’T TOUCH ME
could I also just add that kids from abusive households tend to assume that yelling is directed at them and/or it heralds something bad for them so she’s making extra sure that he knows that this is not his fault and she’s not actually mad at him.
Also look at his reaction, he tenses and looks to Ron and the Twins for support.
Whatever you do, don’t think about the new Muggleborn 1st years wondering why so many of the ghosts are teenagers in modern day clothing and why so many people like to pet the air and they wonder if that’s some kind of magic custom or something and not realizing that they are all petting the thestrals