Im one of the parents that helps out on TSR (married 27 years). When it comes to women, its the classic women are loose and easy if they have too many sexual hook ups. Its an age old classic. She probably thought you would run a mile if she had told you are the start. So she told a fib so her body count did not look so bad; and as time went on, it went from worried she would scare you away from the start, to worried she would scare you away as the fib went on for far too long.
If back then if she had been upfront and said hey ive been with 17 guys, would you have run a mile? Was you back then the type of person who would be appalled by that? Maybe she thought at the time you would be. Im not defending her actions, but trying to see back then her mindset as to why she lied. I do think you should talk to her about it as to why. And as you say, shes been an amazing wife and mother, and she has not been drunk until now. And you have kids too. A definite talk is needed because you need to put this behind you and not have this hanging over you.
You have had 12 years of an amazing marriage, so its worth fighting for, surely?