Open the Hierarchy editor and click on the brain and drag it to one of the hands. If that doesn't work, there is something wrong with either the alien or the brain. Is the brain a figure by chance? To get it to parent to both hands, you need to turn on IK for the other hand, delete all the key frames for all the arm parts (just that one) then set the other hand to be parented to the brain from Object/Change Parent. Then go back and delete all the key frames again. Now the brain should be a child of one of the hands and the other hand is a child of the brain. Simply move the parent hand and both the brain and the other hand should move. Alternatively you can use the brain and make both hands a child of it in the same way as making one. Then when you move the brain, both hands should follow.

Just be mindful that this method often causes joint "popping" if you move it too much.

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