The Vacation by Polly Horvath

From chapters.ca:
When his mother decides on a whim to be a missionary in Africa and drags his unwilling father with her, Henry is left in the care of his Aunts Magnolia and Pigg. Henry's sure they dislike him and he's trying to keep his distance, but that becomes more difficult when Mag decides they should take a destination-less road trip. Mag, convalescing from an illness that makes her look like death, is downright crabby. Pigg, tense from driving, is becoming more assertive and less willing to submit to Mag's whims. And while they poke each other - literally - Henry is finding it hard to keep his resolution.
Complete with her signature cast of eccentric characters, absurd situations, and heartfelt moments, Polly Horvath writes an on- the- road epic like no other!