What is the best way to accurately diagnose Prolong QT Syndrome, and should I take LQT seriously?

My younger sister was diagnosed with LQT after having sudden bout of seizures with landed her in the hospital. With monitoring it was detemined that she need a defiblirator. I have since had an ECG with show a slightly over the normal range QT. I have never passed out or had any symptoms and I am just not sure how seriously I should persue this Prolong QT diagnoses? It just seems that I am going to all these doctor appts but no one can really say that I have LQT Syndrome with out having genetic testing. Besides and ECG, are there other test that can be done to determine if I have LQT.

The long QT syndrome (LQTS) is a heart disease in which there is an abnormally long delay between the electrical excitation (or depolarization) and relaxation (repolarization) of the ventricles of the heart. It is associated with syncope (loss of consciousness) and with sudden death due to ventricular arrhythmias. Arrhythmias in individuals with LQTS are often associated with exercise or excitement. The cause of sudden cardiac death in individuals with LQTS is ventricular fibrillation.
Individuals with LQTS have a prolongation of the QT interval on the ECG. The Q point on the ECG corresponds to the beginning of ventricular depolarization while the T point corresponds to the beginning of ventricular repolarization. The QT interval is measured from the Q point to the end of the T wave. While many individuals with LQTS have persistent prolongation of the QT interval, some individuals do not always show the QT prolongation; in these individuals, the QT interval may prolong with the administration of certain medications.