My W had a brief PA a number of years ago. I never knew about it, and she ended it quickly, but has felt guilty about it ever since. She told me about 2 years ago, on the advice of her C, because keeping the secret was destroying her ability to have a close R with me.
I would think that the stunning part is that she told your family, and then did not tell you soon thereafter - not that it has been 12 years. After the first days of shamefully keeping the secret, it would get only harder and harder for her to tell you - because then she has to explain not only the act, but the delay in telling you.
Have you thought about it that maybe, the fact that she has been keeping this secret for 12 years is a part of what is preventing her from being close to you now? Maybe if she were to find out that you now know about it and you don't overreact and the two of you can talk about it calmly, it would help your current sitch.
I am not at all saying you should back down or let her walk on you, but you were to be strong, rational and open about it, it might help remove a barrier to you getting back together.
Me 42, W 39, S8, S6, S2 M 11y, A & ILYBNILWY 11/08 Walking away from a bad situation.