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?