The difference between you, your dad, your grandfather and your great grandfather is twofold. The first is that you are female, and esterogen, which you produce naturally, protects you against some forms of heart disease
. The second is that since your dad, grandfather and great grandfather's time, HUGE advances have been made in detecting and treating heart problems. The fact that you have a heart murmer which has already been detected, is a step in the right direction. Likely you return to your cardiologist at regular intervals for monitoring, which will help pick up any other problems before they reach a serious stage. And because you are concerned with the problem, you will be watching your diet and getting enough exercise to keep your heart healthy.
Don't be scared -- all the odds are on your side. Just relax, do what your doctors tell you, and enjoy life. Did you know laughter actually helps protect the heart? So look on the bright side of things. Then come back on when you reach 81 and tell everyone how enjoyable your life has been. (I know, 81 is a long way off, but that's the average life expectancy now)