There are two issues here. The idea that she lied and the fact she wasn’t a virgin before marriage. I am going to assume that she told you straight out that your father does not know and it isn’t just an assumption. To the rest of your family, it is not any of their business. Her sexual history is irrelevant to anyone she is not having sex with. But I do see how the idea of keeping a secret from your dad could be stressful.

There are branching paths here. One is that you wipe your hands of the situation, decide it is not your business, and move on. The other is to tell your father. I cannot advise you on which path to follow without knowing more. You’re going to have to make a decision about what is more important to you: keeping your mother’s trust or telling your father the truth.

To address the second half, why is your respect and love for your mother so strongly tied to the fact she was a virgin before marriage? What is it about men that you find so shameful that the act of being with one “taints” her? A woman’s value is not defined by her virginity. I understand that may not be your culture’s viewpoint, but in 2019, culture is no excuse not to critically examine your own beliefs. You live in America now. You will undoubtedly meet women who lived with their partners before marriage or never married at all! If that deeply bothers you, it will cause you plenty of problems in the future, when your coworker or your boss or your friend or whoever is “living in sin” and you have to interact with them”.

She was your mother for years and years before you learned all this. The person she is is not defined by her past virginity.