First I read Foster for the story and it was a moving and beautiful one, though I felt a bit let down at the end. Who was Daddy, why, what, did she go back to the Kinsellas, did she stay with her parents? Then I read it again, in the light of what we find out in the book, and it all made more sense. Of course there could be no resolution - families and emotions don't work like this - but she has found, over the summer, in Kinsella, a Daddy for the first time, and an unswervingly kind and steady older sister in Edna. She'll visit them again, but even if she doesn't, they've awakened something in her.
Straightaway the Kinsellas set down the terms of her stay by assuring her father that they haven't taken her in for her potential as a farm worker. Unsentimental and practical, we know they are good by the delicacy with which her bed-wetting is dealt with ( the 'weeping' mattress). Also Edna ( I think, anyway) deliberately lets her go from the wake with the horrible Mildred knowing full well that she will hear about the tragedy - because Kinsella and Edna couldn't tell her themselves. (How could they, without making her sad or guilty or feeling she was second-best?) What was lovely was that Edna trusted the girl not to credit Mildred's ugly insinuations ('that's what they said happened anyway').
I also like the way the girl's own mother isn't as 'bad' as the father, just overwhelmed beyond belief. She has taught her daughter the names of plants and herbs, which means there was a time she brought her for walks. The disorder of her parents' house, after Kinsellas' house, is shocking to the girl. The section that describes all the jobs she and Edna do together, day after day, makes us see that domestic order is, itself, a kind of love.
By the way, small point; another reviewer set the book in the 1960s/70s, but the author deliberately, by mentioning the hunger strikes, lets us know it was 1981.