I think you need to give it a little more time....Sierra might have been use to being the dominant one in her other homes and jake has been around your house for a while...so Sierra comes in thinking she's the "top dog" and Jake thinks otherwise. Its a power trip for both of them. I think reprimanding and separating them when they fight should eventually get the point across. Eventually they should come to a mutual agreement on dominance and things should settle down. Its a completely new situation for both of them. Just be sure you show Jake plenty of affection to show that you still love him, but also show him that now he must share your affection (and his toys) with Sierra. Show Sierra the same affection so she knows that you accept her. I hope this helped....Good luck and keep us updated!