If she forged his signature she has committed a criminal offence (& presumably she got someone to witness the forgery, or else forged witnesses signatures as well?)
As you say your dad cannot hold a pen, another possibility is that she has power of attorney. If so, she could effect the transfer. But the power of attorney would normally be an enduring or lasting one, & various relatives (probably including yourself) should have been notified that it was to be made effective.
What exactly did she ask the nurses to witness? If he can't hold a pen what were they supposed to be witnessing, as it would not have been him signing?