Do we know that she is happily married?
Not sure what religion has to do with it, Catholic girls are some of the kinkiest. lol
If she comes to work at the school, there we may need to punish her for interfering in her daughter's education or something. It may, or may not include her daughter at the time, and even if it does, that would not mean anything for incest, but I can make it make sense, believe me.
Have her busting in on you while you are giving the girl private tutoring about evolution and such that she missed, insisting that you are wrong and some mumbo jumbo, bla bla bla. Obviously, this woman would also benefit from these lessons, and needs to be punished for interrupting her daughter's lessons. You are doing this for her benefit after all, and what mother would not do everything possible to help her daughter? 'You are not a bad mother, are you, Madam? No, no, no, of course not...'