Our sister, co-trustee, removed another sister, co-trustee, from family Will with alzheimers' mothers signature, legal? penalty?

You do not mention if your mother is still alive. I assume you mean, by the family will, your mother's will. Or, did you mean your parents’ trust? You mention trustees. If there are sufficient estate assets, you may wish to consult and retain an attorney to review the facts and file any necessary petitions. If the only modification to a family trust was to remove a co-trustee, IMO, that is probably a no harm, no foul situation. The remaining trustee still has fiduciary responsibilities to both your mom, assuming she is still alive, and to any other beneficiaries. If the trustee is not living up to her responsibilities, then a petition to remove her may be appropriate.

Alzheimer's does not necessarily constitute legal incapacity. Although absent a doctor's statement, presence of an ombudsmen, and a videotape of the signing process, it is easily challenged. You need to retain a trust attorney to look into it if you suspect any improprieties.

I don't understand the Will/Co-Trustees part: is there a Testamentary Trust? Created by Dad's Will? Did Dad have a Special Needs Trust for the sole benefit of Mom during her lifetime, and then the remainder to various children? Did a blended family became unblended at the first death? If there is a LOT of money involved, you can all duke it out in open Court, but ZI'm wondering if that is really necessary if what is needed is even=handed control. Would filing for Conservatorship of Mom be workable, with one adult child from one side handling the Estate/money and one from the other side handling the Person/Care Plan? Conservatorships are expensive too, though, so if the issue is about control, then hiring a Geriatric Care Manager or Gerontologist to look into the adequacy of the services being provided might be a lot less expensive and much more satisfactory of an appropriate result. AZ is not an automatic bright-line for 'has capacity vs. doesn't have capacity', so with the holidays coming up, my hope is that you can all use this time of good will to identify more clearly exactly what the problem is; is it objection to having no input? That can be easily managed because there is a LOT to do in any fiduciary administration.