The twin's mother is inside the phone telling it what to do? Or the person you replied to is talking about technology finding it difficult to distinguish, and not a close family member?
My point is that twins are only difficult to distinguish for people who do not know them. For family and friends it's easy, which means that there are substantial differences even for faces of twins.
The face detection technology will be able to recognize those differences as good or better than the twins' family and friends. If it does not now, it will eventually.