How do you forgive for something like that? Well you realize that it hasn't affected you in anyway, realize that they still love you like they always have and that its between the three of them whether or not they can forgive one another for what went down. None of them; your mother, your father or your grandmother love you any less even though your father and grandmother made some pretty fkd up decisions with their lives.
You say you found out two years ago but the affair happened 12 years ago:
Does that mean your parents have been separated/divorced since the discovery?
Have you been unable to forgive for two years?
Have you thought about getting some councilling to help you?
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion