I am by no means a lawyer but I would be more concerned about the stepmother hitting my child than the sleeping arrangements.
I can speak a little from experience with a situation similar to yours where a stepparent felt the need to use corporal punishment on my children. As a stepparent is considered a legal stranger and I am sure quite possibly the stepmother was given free rain from the dad to punish the children as happens in most cases. However, because they are a "stranger" in ever sense of the word, the police look at it differently than CPS. Where CPS may not have a problem, the police would consider it assault on a minor. I was informed that the process is as follows: file a police report (which can actually be done in your state when the child visits as a courtesy report) that is then sent to their state, they go to the other parents house, investigate and if allegations are proven to be true, the DA takes over and prosecutes.
With all this in mind, I would suggest you consult your attorney. Maybe its a matter of using this as leverage to prevent future assault.