If she has "some" dementia, she might not have capacity to be a joint executor together with yourself.... if this is the case, she could voluntarily refuse the appointment as joint executor, leaving the task to you. If not, then you would need to consider if you want to attempt to force the issue, and use the medical opinion of the doctor as expert evidence to have her removed as a joint executor.
ADMINISTRATION AND PROBATE ACT 1958 - (VIC) SECT 34
Discharge or removal of executor or administrator
S. 34(1) amended by No. 110/1986 s. 140(2).
(1) Notwithstanding anything contained in any Act where an executor or administrator to whom probate or administration has been granted whether before or after the commencement of this Act or where an administrator who has been appointed under this section or any corresponding previous enactment—
(a) remains out of Victoria for more than two years;
(b) desires to be discharged from his office of executor or administrator; or
(c) after such grant or appointment refuses or is unfit to act in such office or is incapable of acting therein—
the Court upon application in accordance with the Rules of Court may order the discharge or removal of such an executor or administrator and also if the Court thinks fit the appointment of some proper person or trustee company as administrator in place of the executor or administrator so discharged or removed upon such terms and conditions as the Court thinks fit; and may make all necessary orders for vesting the estate in the new administrator and as to accounts and such order as to costs as the Court thinks fit.
S. 34(2) amended by No. 110/1986 s. 140(2).
(2) Notice of such application may be served if the Court thinks it necessary upon such persons as it directs.
(3) An executor or administrator so removed or discharged shall from the date of the order cease to be liable as such for acts and things done after that date.
(4) Upon such appointment the property and rights vested in and the liabilities properly incurred in the due administration of the estate by the executor or administrator so discharged or removed shall become and be vested in and transferred to the administrator appointed by such order who shall as such have the same privileges rights powers duties discretions and liabilities as if probate or administration had been granted to him originally.