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.
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.
EKG. Yes, it can be hereditary. A simple electrocardiogram (ekg or ecg) can measure the length of your qt interval.