The young siblings entered the final leg of the race in pole position with a six-hour lead over their nearest competitors, but at the start of the episode they decided to fly home due to a "family emergency".
In a letter they sent to the other three pairs in the competition, they wrote: "Dear guys, we're sorry to say a family emergency has meant we've decided to leave the race.
"Wishing you all the best and good luck! Catch you all back on home soil."
Providing an update on social media, former racing driver Billy, 24, explained that a family member suffered an accident but has since recovered.
"Bonny Monger and I were gutted to have to leave the race," he told his followers.
"A close family member had a bad accident so we had to get home. Thankfully all okay now. We loved the journey!"
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