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?
We see evidence of it throughout the series too. She knits him a sweater like she knits for all of her biological children because Ron mentions to her offhand that he’s not sure if Harry is expecting any Christmas presents, and this is before she’s even really gotten to know him. She just knows that he’s an orphan, and that he’s had a hard life. Whenever Harry stays with the Weasleys, either at the Burrow or Grimmauld place, she always feeds him extra helpings of food, because she knows that the Dursleys would starve him. She opens her home to him during school breaks, her and Mr. Weasley come to be his “family” before the third task of the Triwizard Tournament, she throws him a large coming of age birthday party, even in the midst of being stressed out by preparing for a wedding, and the world going to Hell in a handbasket. Molly also has the distinction of being one of the few adults in the series who is intent on shielding Harry from having to grow up too quickly. It just breaks her heart that he doesn’t get to be a teenager, and we see her over and over again absolutely deadset of preserving what scraps of his youth that she can. And the thing that above all proves to me the most just how deeply Molly cares for Harry? She tells Sirius that Harry is as good as her son, and she is absolutely passionate and furious when she says so. I just have so many feels. JK might have been squiffy about giving Harry a definite father figure, but she for certain gave him a solid surrogate mother, it makes me cry sometimes if I think about it too much.