Is blood pressure value of 126/66 a cause for concern?
I would recommend as follows:
Welcome and thank you for asking on HCM!
I understand your concern and would explain that as long as you have no symptoms of coronary artery disease (chest pain, difficulty breathing,etc.) and these blood pressure values, everything seems to be OK.
A family history of cardiac disorders does not mean for sure that you are going to suffer from cardiac disorder in the future.
You should know that there are many other factors which influence in your cardiovascular risk, besides your family history: age, unhealthy lifestyle, hypertension, diabetes, obesity, drug abuse, etc..
In such situation, based on your age, I would recommend periodically scheduled follow ups (6-12months) by your GP with:
-chest X ray study
-blood lab tests (complete blood count, fasting glucose, kidney and liver function tests, blood lipid profile, blood electrolytes, etc.) to check that everything is OK.
Meanwhile I recommend continuing physical exercise, use a healthy diet (low fat and sugar food and rich in omega 3, low caffeine and salt intake), avoid heavy drinks and smoking and reduce your weight if you are overweight.
Feel free to ask any other questions whenever you need!
Hope to have been helpful!