"I have a sister who is a textbook Histrionic. All of my life (I'm 24 now and she's 28 ), I knew something was wrong, but I thought "she's just a shallow person." I finally figured out she has HPD and I was floored by how much she fits the criteria."
A lot of the time people with Borderline Personality Disorder exhibit all of the traits of an HPD woman.
Have you looked over any BPD criteria to see if your sister may fit that as well?
I ask because generally HPD's only really hurt people they are in relationships with.
Everyone else they can be an energy drain on though with their constant attention seeking.
But anyway a BPD goes through cycles of calm, raging and other things that really warp a person whose dealing with them's reality, frustrating that person and making them crazy in the process because nothing they do can make the situation right seemingly.
"The worst part is that no one in my family seems to see it. They are too concerned with their daughters fighting. Even therapists are wooed by her personality and cannot see what she is doing to herself and to her family."
Cluster B Personality Disorders of which HPD is a part are all sociopathic in that a person with one of them plays a likeable character very well and does it so convincingly because while they are playing it they believe that is who they are.
This is why their manipulation works so well even on trained professionals.
Some professionals I've come to read about don't really know much at all about Cluster B Personality Disorders which is very unfortunate for not only a person going in for treatment wanting to find out what they have and overcoming it or like in your case your sister not being diagnosed with anything because either the therapist didn't know about that condition or because her manipulations were so good he couldn't tell.
"After years of trying to deal with her, I've given up. She started a heated argument over nothing (usual melodrama) and declared that I am dead to her."
Again I'm getting stronger BPD vibes with your sister than HPD.
"Even when I respectfully asked her to not get my parents involved, she insisted thus giving them extreme stress. My parents are pressuring me to "solve this" by flying to LA (where she lives) and seeing a therapist for 3 hours, as if that can solve it. Since my sister is the one who demanded I see her therapist, I chose not to give into her terrorism."
Good job on your part.
You have to be very firm when dealing with someone with a personality disorder.
Clear boundaries must be established and you should never give into any demand.
At best come to an agreement on things but don't be the one to always give 100% while they give nothing in return.
"Anyway, my question is, how do people deal with such "hardcore" histrionics? Is it best to just pretend I never had a sister? I've realized we can never be friends, so I tried to just be cordial, but even that wasn't good enough since she wanted more attention."
Is she so toxic to your sense of well being in life that it would be better without her in it?
If the answer is yes than you must get her out of your life.
If it is no then obviously you will just have to deal with her the best you can.
"Is it always all or nothing? And if somehow my parents coerce me into having a "relationship" with her, how do I deal with her?"
Untreated HPD's and BPD's are inherently selfish.
HPD's because they're so wrapped up in their own emotional dramas they don't have time to worry about their effects on anyone else and BPD's because they don't stop themselves for a moment and consider not doing something that would make them feel good but harm someone else.
So yeah to deal with her untreated it will always be all or nothing as far as you needing to give her attention but you can protect yourself by knowing her better than she knows herself and setting limits on the amount of attention you give her.
In closing I highly recommend the book Emotional Vampires to you.
I think it is only $8 dollars on Amazon.com and it has excellent sections on dealing with the various histrionic types to make them less of a headache for you.