I am a nurse in acute care, so apply my method as needed.
Usually when family members begin "ordering me around," there is no doubt that they have multiple, rapid-fire requests/orders given at a time, usually none of which are urgent. Assertiveness works wonders to getting family members to chill out. At this point, I usually say, with direct eye contact and an authoritative demeanor, "You have multiple requests, but I can do only one thing at a time. Let me address this first, then we'll talk." Then, I *do* talk to them! They might have never developed trust with the previous nurse or tech. They might be extremely anxious about their family member. They might feel a loss of control. Whatever it is, I try to get to the root of the problem and fix it head on. As jadelpn stated, do they understand the plan of care, which when explained, often takes care of anxiety and loss of control issues? At first, it might seem to be a time waster to discuss things with them for 10 or 15 minutes, but I have learned that the minutes I spend talking to them does wonders to stop the behavior, most of the time and at least during my shift.
You might have to repeat yourself because some things are so engrained in the person's mind that they don't get it the first time. I had one patient and his brother who wanted to know how long he would be in physical therapy/rehab as I was getting ready to transfer him to our rehab floor. Although I stated form the get-go that it depends on how quickly the patient met his rehab goals, I had to say it no less than FOUR times!
You'll always run across the family member for which this approach simply does not work. In that case, I have wasted my time with the discussion, but then I set limits and may even write a "rounding time" on the white board. Even if it does nothing for the family member, it saves my sanity and allows me to care for all my patients, not just *that* one.