Child hitting parents

Hello everyone, My wife and have a sibling group of three older children that have been placed with us for two weeks. Two boys - age 9 - are fraternal twins and a girl - age 6. I was wondering if anyone has dealt with a child that was abusive to everyone in the house. One of our boys has hit or kicked my wife and I approximately 6 times in two weeks. He has thrown things at us. He also hits his brother and sister pretty regularly. His hitting incidents usually occur as a response to some other form of discipline ( go to bed, don't take my keys to the car, stop hitting your sister or brother) In addition he does talk back a lot and when he has not hit us he likes to make a threat with his fist or an object. While the hits haven't hurt physically I am concerned about a potential escalation in behavior. We have tried grounding and taking away of priveliges. Any thoughts? How concerned should I be about this behavior? Thanks, Jeff PS - Yes, I am very concerned.