The 25-year-old could have been lining up alongside Leon had he had not made the bold move to become the Catalans' first English signing and is now hoping to shake off a hamstring injury to play.
Leon, who left Bradford Bulls in 2005, turned down the chance to return to Odsal when his contract with rivals St Helens expired at the end of last season.
Karl said he was surprised by his brother's decision to leave England but, after a torrid time of his own, is hoping he can make his mark back in West Yorkshire.
"I think he had the opportunity to come back but he chose something different, a new challenge, maybe a more difficult challenge with the language barrier," said Karl.
"For Leon to move out of his home town is something I thought he would never do but he seems happy."