That's super simple: Believe them.
It doesn't matter if you understand or not, just believe them. If they tell you they hurt, believe them. If they are tired, believe them. If they cancel plans because they don't feel well enough, believe that it's truly because they don't feel well enough.
As fibromyalgia patients the largest part of our battle is simply to be believed.
If you need help in order to believe, then get it -- this could be by reading books or articles, asking questions, watching videos, and so on.
Or, you could simply choose to believe every word your family member says is true.
But that is far and away the best thing you can do to help.