How can I protect my mother and her estate from my deceased brother's girlfriend?

If your mother's doctor will certify that she's not mentally competent, you should hire a lawyer and file for guardianship.  That will accomplish two things that aren't in place, now, with the power of attorney, although your role in your mother's life will be pretty much unchanged:  first, your mother will become a ward of the court, and whether you're around or not, she will be "in the system" where her rights can be protected.  Second, unlike the power of attorney, a guardianship can't be revoked by anything your mother signs;  she'd have to come into court and move to end the guardianship herself.

I think you can probably invalidate anything your mother would sign now, if the doctor agrees she's incompetent.  But I'd recommend the guardianship:  "vultures" are best dealt with before they create an actual problem, rather than after.