I have encountered some mental health professionals with non-standard ideas. No one ever said they "didn't believe in bipolar disorder", but I did go to a psychologist once who said he didn't believe in personality disorders. I don't have a personality disorder, but I think it unfair to those who do that they wouldn't have their challenge recognized by him.

If your sister's doctor treats her with appropriate psychotropic medications, then I guess it doesn't matter what he does or doesn't believe in. However, I would personally steer clear of the guy or at least ask him what the heck "HE" thinks is going on, if he doesn't believe in the illness. If he said something slightly reasonable, one could just agree to disagree. If he said something far out in left field, it would be good to know.

I wonder if he "believes in" schizophrenia or even anxiety.