The Magus Sisters have nothing to do with the Trials and are dependant on your acheivements in the Remiem Temple and the Monster Arena, so even if you can't obtain Anima, these are still available
Actually that's wrong.

You cannot get the Magus Sisters without getting Anima because you have to fight Anima at Remiem temple. You have to fight ALL the aeons including Yojimbo and Anima to get the Magus Sisters.

Not that you need them. The final boss is a pushover.