Although I have never come across a study directly on point, one could deduce that borderlines are, on average, highly intelligent individuals from related empirical evidence. For example, borderlines have been shown to be "logical" and "highly perceptive" individuals. Both of these traits factor into a person's IQ. (I myself have been diagnosed with BPD and bipolar II and my last-tested IQ score was 157.) Studies have also shown that, in the overwhelming majority of cases involving a sexual relationship between therapist and patient, the patient had been diagnosed with BPD. This has been attributed to the intelligence level of borderlines (as a theory), along with charisma, seductiveness, physical attractiveness and general high functioning.

The trouble with the generalization, however, is that BPD comes in far too many forms to responsibly rule out BPD based on any one factor alone. To illustrate, the most recent therapist I saw "undiagnosed" me with BPD based on my maintaining "too many" stable relationships. The psychiatrist I saw the same day likewise dismissed the diagnosis because I was "too nice." She added that a borderline would also not be likely to attempt suicide (as I recently done) without giving anyone advance warning. All of these assumptions are the sad result of an epidemic of overgeneralizing and stigmatizing borderlines. I may not fit the profile perfectly, but the diagnosis is legitimate - the same could be true of your family member.

My advice would be to get your relative to a more open-minded and informed clinician so that you don't have to worry about him/her receiving poor treatment based on an incorrect diagnosis.

That's just my two cents though. I do hope everything works out okay for you and your relative.