Dear dflaucho,
You are hurt, there is no denying that. And worried, and ashamed and truly shaken to the core of your being.
But as a very old friend of mine would say "Has anybody died? Broken bones? Spilled blood? Has anybody lost their home or their livelihood? If not, then let us sit down and think about this."
Once you think about it, you will see that nothing has changed, only your perception of it. Everything that was, still is and will be tomorrow.
I am reading in your post an underlying accusation of infidelity. Given what you have told us and your grandfather's reaction to your father, I think that there is a very good chance that your grandmother had been raped. That is not your grandmother's fault, nor your grandfather's. There is hardly a family in the world in which it has not occurred. You have been unlucky to possibly discover it.
These situations are always troubling and can play on a mind for years. I truly hope that you are able to make peace with your ancestors.
Regards
Chas