It's pretty commonly accepted that we rail against that which we see a lack of in ourselves. While I don't appreciate hypocrisy, I do feel some pity for your mom, who I think obviously is demonstrating some remorse or shame over her own actions. For what it's worth, I personally see no harm in premarital sex, and oftentimes our own desires and lifestyles come into contact with our philosophies, creating a lot of unnecessary conflict. Not that your mom should throw her philosophy to the wind, of course, but in my view, would be better off reconciling her own life with it.
Love you both.