A will although registered can be challenged in the court of law. The mere fact that a will has been registered will not, by itself, be sufficient to dispel all suspicions regarding it. A registered will may not be the last testament. A new will made, even if unregistered, if valid, will trump the registered will.
Was your mother alive at the time of making this second and last Will?
Did you recognise or confirm the signature to be her signature or not in the subsequent Will?
Do you have any doubt about the subsequent Will, if so you can challenge the same through court of law if they are trying to enforce the Will.