Goldie is MAD and it's all about her baby brother.
I would NOT introduce this book to a toddler or young child (even one who already exhibits strong, negative emotions and words) as it will either introduce or reinforce downright awful behavior. There are much better books addressing anger in children.
Goldie yells at her demur brother, "I hate Nicholas!" and although she does receive a time-out, the book doesn't delve deeper into her emotions or show acceptable outlets for them. She "thinks" about it. In the end, Goldie realizes she likes some things about her baby brother and we see she still needs to work on her patience with him but she certainly hasn't reformed or understood why her behavior is unacceptable.