SOLVED. YA funny book about a girl whose brother has imaginary friend Henry. [s]

[Lorraine]

A girl is on a road trip with her embarrassing family. Her annoying younger brother has an imaginary friend called Henry. During an argument while travelling, she opens the car door and says, “Oh oh, Henry fell out.”
Her little brother gets hysterical and her family gets mad at her, so the dad turns the car back to collect the imaginary friend!
She narrates that she then has to spend the rest of the trip sitting between someone (grandma?) and “Dead Henry”.
I heard this excerpt read in school in 1995. I’m in Australia. I considered that it might be from the “My Life is a Toilet” series by Gretyl Killeen, but that main character only has sisters.

[Ayshe]

My Diary: by Kylie Mole maybe?

[Lorraine]

I think that must be right, Ayshe! Thank you very much! Now to find a copy... :)