For much of the story, Sal doesn't trust Margaret Cadaver. Egged on by her friend Phoebe, who makes Margaret out to be a mad ax murderess who killed her husband and buried his body parts in the garden, Sal is deeply suspicious of this kindly nurse and her intentions.
Sal's especially suspicious of Margaret's intentions towards her father. She's got it into her head that Margaret has romantic feelings for Mr. Hiddle and is somehow trying to take the place of Sal's mother. And so, despite Margaret's best efforts to be nice, Sal cordially detests her, at least until she finds out just what a nice lady Mrs. Cadaver really is.
Sal's suspicions of a burgeoning relationship between Margaret and her father are fueled by the different way that Mr. Hiddle acts at Margaret's house from how he behaves at home. Whenever Mr. Hiddle's at Margaret's house, it's noticeable just how happy he is, smiling and sometimes even laughing.
But when he's at home, it's a different matter entirely. There, Sal's dad can usually be found sitting on his bed, staring at the floor, reading old letters, or looking at the photo album. Poor Mr. Hiddle appears so sad and lonely at home. Sal doesn't want him to feel this way, of course, but at least when he's sad she knows that he's remembering her mother.