My sister died of sudden acute myocarditis which the doctor said was related to long qt syndrome. Is this hereditary. Can I be tested?

Yes and yes. This disorder can be due to a number of genetic defects. You can get tested by an ECG and it can be treated. See your doctor and check this site for more info. http://www.mayoclinic.com/health/long-qt-syndrome/ds00434.
Long qt. long qt syndrome and myocarditis are 2 different entities and not related to each other. I am an expert in the field you can consult me or one of us if you need more detailed info. www.healthtap.com/volkantuzcu.
EKG. Yes, it can be hereditary. A simple electrocardiogram (ekg or ecg) can measure the length of your qt interval.