Mrs. Susan Sowerby is basically the 1911 Muggle version of Mrs. Weasley: She is a mother to a big family, and she takes to her role as mother with a lot of heart, enthusiasm, and hard work. In fact, Mrs. Sowerby has twice the job of Mrs. Weasley because she has twelve kids rather than a mere seven.
Despite the challenges she faces as a poor mom on the Yorkshire moors, both Martha and Dickon have nothing but praise for their mom. As Martha says of her mother, "Well, she's that sensible an' hard-workin' an' good-natured an' clean that no one could help likin' her whether they'd seen her or not" (7.20). In other words, Mrs. Sowerby is so sweet and hard-working that she's impossible not to like.
Mrs. Sowerby's amazing maternal instincts make her appealing to Mary and Colin (both of whom are, of course, missing mothers of their own). Colin gives Dickon permission to share the secret of the walled garden with Mrs. Sowerby, since he trusts her not to tell everyone in the world about it.
And so Mrs. Sowerby is the first adult they actively invite into the Secret Garden. And without breaking her word to the kids, Mrs. Sowerby writes to Archibald Craven to say that, "I would come home if I was you. I think you would be glad to come" (27.24). It's with Mrs. Sowerby's gentle encouragement that Archibald Craven comes home to reunite with his son, bringing the story to a happy ending.
Mrs. Sowerby is the novel's ideal mother in some ways: She has none of the physical fragility of Lilias Craven and none of the vain selfishness of Mrs. Lennox. Her devotion to her kids makes her seem as saintly as Dickon, with his amazing attachment to nature and the wild moors.
In fact, she appears to confirm Mary's fantasies of what it would be like to have a real mother: "When Martha told stories of what 'mother' said or did they always sounded comfortable" (6.4). Yet, as with Dickon, we could add that Mrs. Sowerby isn't totally three-dimensional as a character. She is so good and sensible and self-sacrificing that she never seems real in the way that Mary or Colin—or even Mrs. Weasley—do.