Just another story as I was in the same situation as the OP, just I was the child who had no clue.
I was in 4th grade when I figured it out, I found out on my own. We had these censious cards to fill out, it wanted to know if either the SP or BIO parents were ever in the service.
Well when I read mine for whatever reason it said (the man I thought was my DAD) was my Step-dad. I say the man I thought, only because he wasnt the greatest in the world, in fact he was down right horrible at times. He did things to me and my brother that were completely horrible and never to be fogiven for.
My mother lied to me till I showed her how I knew, from that ppoint forward, I have not forgiven her, she wouldnt talk to me about it, made me not talk about it to anyone at all. My grandmother had pictures of their wedding that I had, my Mom took them and threw them in the trash.
When I finally decided enough was enough and I moved out of the house because of things that were being done, I found my BIO DAD, ( I was 23 when we finally met) it was rough and heartbreaking at first but has gotten better and I miss him very much and wish he was closer to me and found out my mom had in fact lied about alot of things to me in regards to him.
So, PLEASE be careful on how and when you do it, but when she does find out, let her talk about it and let her vent/cry/or whatever she needs to do when she understands the story.
Good luck and as long as you show her support and honesty, it will be ok.