Mae and Angus love Winnie like their own daughter, and Winnie loves them back as she would a surrogate family.
At first, Winnie isn't sure what to think of the Tuck family. Her logical brain is telling her that she has been kidnapped by criminals, but her emotional brain is telling her a different message. She feels safe with the Tucks. She feels as if they care for her. She never feels like she is in danger. After spending the night at the Tucks' home and having two meals with them, Winnie tells the reader the following:
Perhaps they were crazy, but they weren't criminals. She loved them. They belonged to her.
Winnie still loves her family, of course. She just loves the Tucks in a different way. They give her a sense of freedom and belonging. They don't micromanage her the way her parents do.
As for Angus and Mae, they love Winnie right back. They love her like their own child. That's made clear the night that Winnie sleeps at their house. Both Mae and Angus come out in the middle of the night in order to check on her and make sure she is comfortable.
He looked uncertain. "Well . . . but if you want something, will you holler? I'm just in the next room—I'd be out here like a shot." And then he added, gruffly, "It's been quite a time since we had a natural, growing child in the house . . ." His voice trailed off. "Well. Try to get some sleep. That sofa there, I guess it ain't the kind of thing you're used to."
The other evidence that supports Mae and Angus' devotion to Winnie is their protection of Winnie from the man in the yellow suit. Mae is willing to physically stop the man in the yellow suit from taking Winnie. She clubs him over the head with the butt of a shotgun. The blow ends up killing the man. Mae wouldn't risk her own life if she didn't love Winnie.