She is particularly good at depicting siblings and families.
In From the Mixed-Up Files of Mrs. Basil E. Frankweiler, she shows a brother and a sister who mostly see each other as annoyances becoming a team (while never giving up their arguing). Silent to the Bone is a rare example: A book about tensions and problems in step-families where there is no bad guy (well - there is a bad guy, but not someone who is part of the families). Father's Arcane Daughter, one of her more obscure books, doesn't hold back in portraying the protagonist's resentment of his younger disabled sister - while still making it clear they are deeply bonded.
Looking over those books, I notice all of them focus on brother-sister relationships - both sets of half-siblings in Silent to the Bone are brother-sister pairs.