Early 2000s movie , maybe Tragicomedy

I saw it in 2016 , it is based on a book. I barely remember it , but its about a drugged mother that is prescribed all drugs from her psychologist/psychiatrist (not sure) and takes her son to live at the doctors family , the family has three daughters (if i remember correctly) and the house is all messy (The christmas tree still there) , there is a scene i remember clearly where one of the daughter eats the family pet (not sure if its a cat).

The daughter ate the pet??? Is it horror?