D.B.
Well, Sera's not allowed to run across the parking lot either. So Ava and Sera can sit in time out together. Or Ava can show Sera what happens when there is scratching and kicking since apparently Sera doesn't listen to you, only to Ava.
I would get some professional help on this. Imaginary friends are very common in kids, but alter egos are often created after trauma as a way to give voice to feelings. You should get some good advice from a child psychologist to develop strategies for handling it and for learning how much to allow and how much to clamp down on. Talk to the pediatrician for advice and referrals.