While JHS/EDS can be genetic (although there are spontaneous forms of it, so some people have it with no family history of related disorders whatsoever), the reason that there is an association between JHS/EDS and POTS is actually ... well, mechanical. As I'm guessing you've come to understand, JHS/EDS causes excess "stretchiness" not just on a gross scale—in the joints, etc.—but on a fine scale even down to tissues within the body such as blood vessel walls in some individuals. If the blood vessels are too stretchy, blood is easily able to pool in the dependent limbs (the legs and feet when standing upright) as well as the abdominal bed (which you will sometimes seen called the "splanchnic bed" in technical medical literature), which is a dense web of blood vessels in the abdomen. The more the blood pools, the less blood returns to the heart and is able to be cycled to the vital organs, especially the brain, lungs, and the heart's muscle itself.
While the *reason* for POTS symptoms may be slightly different in other patients with POTS than in patients with JHS/EDS (which is to say, not all patients with POTS have overly stretchy vessels—this is just one of many things that can cause this syndrome), the outcome is the same as it is for other POTS patients. Thus this "functional" thoracic hypovolemia when upright causes the heart to try to compensate for it by beating faster to pull the blood back up to itself (causing the well known tachycardia of POTS). If the tachycardia is insufficient compensation, there won't be enough oxygenation to the brain and symptoms such as dizziness, nausea, lightheadedness, tunnel vision, and headaches may result. Ultimately, if the compensation is not enough, the person might faint.
So that's the connection between JHS/EDS and POTS basically. There are a LOT of different medications that can possibly work to treat POTS. Which ones have you already tried which haven't worked? (And in which combinations with each other?) What other "helps" have been tried (such as abdominal binder, compression stockings, etc.)? What type of physical therapy was tried (aquatic tx, cardiac rehab, weight training, pilates, aerobics, etc)?
Here's our health page on treatments. This includes treatments for various types of dysautonomia, so not all would be appropriate for POTS necessarily. But it may give you an idea of what's out there as a starting point:
http://www.medhelp.org/health_pages/Neurological-Disorders/Dysautonomia-Treatments/show/171?cid=196
Best,
Heiferly.