The truth is, no matter what you say or do, if the woman is superstitious enough to actually consult a fortune teller, then she's going to take that person's word over yours, her daughter's or anyone else's any day of the week.
Your best bet is to play it nice and clean. Try to talk to the woman. If she doesn't accept that, just continuing seeing her daughter on the side and work your way back into the mother's good graces.
People who consult fortune tellers generally have great faith in them. Keep that in mind and try not to come across as pushy.