So sorry, OP. This is a very tricky one and must have blindsided you completely.
My husband had an affair, and because we were going to try to reconcile, and also at the time it came to light, one of our older children was in the middle of very important exams, I decided it was in their best interests not to be told.
I did not do it to protect him, I could have put it on billboards in ten foot high letters at the time, I was so hurt and angry. Rightly or wrongly, I did it to protect them.
I thought that there was no point in ruining my older child’s future, damaging the relationship of all our children with their father, and damaging the family as a whole, if we were going to stay together. Except for the very obvious, he has been a very good father for nearly twenty years. He was not a good father during the affair, whatever anybody says about such things. I have read on MN before that you can have an affair and be a good parent. Not during the affair you’re not, as far as I’m concerned, you’re putting your needs first, not theirs as they deserve. He risked their well-being, future and mental health for his own selfish kicks at the time. Not exactly in the Parenting 101 manual. He has been a good father before and since.
It initially took acting of a Meryl Streep standard from me, because mentally I was on the floor at the time, but it was worth it. We are happy and it all worked out.
However, once I decided to do this I knew that however I might feel, I could never, ever divulge this to them as it would in all probability feel like a double betrayal. If their father ever does anything like that again I am divorcing him and in those circumstances I would tell them the truth and hope that they would forgive my silence and see that did everything I could at the time to keep our family together. Tell my children in any other circumstances? Never. Totally unfair.
I understand your mum’s secrecy initially, but not her decision to finally tell you. Having decided to be silent, unless unavoidable circumstances dictate, I think she should stay silent. What was her motivation?
How she can’t see that this would be a twenty megaton bomb exploding in your head is beyond me. I’m so sorry for all the hurt and confusion you are no doubt feeling with no outlet at all. You really do need to talk to someone about this, you can’t keep the mental fallout to yourself. Please take good care of yourself XX