My understanding is that her father’s genetics play an equal share here. Which, if I’m correct, would reduce her infertility risk by 50% from that of her mother and aunt. I’m not sure so I think you need to discuss this with a geneticist or at least read up on it. There’s also always a risk someone will be infertile, so there really are no guarantees with anybody else either.
Additionally, if you’re concerned enough to drop the shidduch, find a way to confirm more details instead of assuming the worst. Besiros tovos!