It's pretty much in the title. Fred is the only one of the two to have magic. It is why they get poor school results and don't fight together in the final battle of Hogwarts. This is how it all started...

Alternative ending to Chris-Crossed. Wyatt breaks free from Bianca's hold just seconds to soon and stops Chris from returning his powers. Knowing there is nothing more he can do, Chris instead saves Bianca by joining with Wyatt. When he doesn't return, the sisters grow even more determined to get him back.