A low resting heart rate (below 50 is called bradycardia) is, normally no problem, as long a you do not have symptoms of dizziness, fainting or shortness of breath.
However, it is up to the cardiologist to determine if it is harmless or not.
Bradycardia does not necessarily mean that you have a thickened left ventricle, so your situation may be quite different compared to your sister's.
Still, sudden cardiac death can be caused by bradycardia so it is of importance to get the "green light" from your doctor.
take care,
Ger