Hi @kwkm. Sorry your sister is dealing with this. Mayo Clinic is one of the best places to seek care for HCM in the world. You can start learning about HCM here:
https://www.mayoclinic.org/diseases-conditions/hypertrophic-cardiomyopathy/symptoms-causes/syc-20350198
Basically, it is usually but not always hereditary, and it consists of a heart that is thicker than normal. The thickness causes the heart to be stiff and can cause blockage of the blood flow leaving the heart. It can also cause arrhythmia.

Another couple of places you can read about HCM is a blog that I actually write and moderate:

http://Www.hcmbeat.com/home

And lastly, the HCMA is another good source of information:

http://Www.4hcm.org

There are several of us with HCM around here. Feel free to ask any questions. Though HCM has had a bad reputation for many years, it is, these days, a highly treatable disease which is compatible with a normal life span. The key is to be seen at a center that sees lots of HCM patients and has an expertise in HCM. Not every place does, and even though by numbers it should be very common, most doctors are just not that experienced with treating it, and the treatments are often quite different than with regular heart disease.

Wishing your sister all the best.

One bit of advice - General medical websites tend to paint a very scary picture of HCM. Please go to the links that Cynaburst mentioned as they are experts and can give you honest, practical and patient-focused advice. Please understand that HCM is now estimated to affect 1 in 250 people. 80% of the population with HCM does not know it, and will likely not know it as it doesn't affect their life enough to warrant a doctor visit. With proper care - folks with HCM live just as normal and long a life as the general population. And its really key to be seen by someone experienced with HCM treatment. Even in large cities with excellent healthcare systems, very few cardiologist have no more than a handful of patients - and this isn't enough for them to learn about the fickleness of this disease. At places where the focus is on HCM, they are often a sub-set clinic within an existing medical center (Like Mayo's) and the majority of their clients have HCM or one of its similar genetic conditions.
There is a lot to learn and it may seem overwhelming at times - I often say "that you've have had HCM your entire life and you really don't need to focus 100% on it" - take your time, ask questions, vent, share and know you have a good support system here.