This is a complex issue because the law in this area is a lttle uncertain.
The first thing to consider is whether the Wills that they made together are "mutual" Wills. These are Will made by a couple at the same time which state that they cannot be revoked so it is not possible for the surviving partner to change their Will after the death of the other.Applying this to your situation if the Wills made were mutual then this should prevent the step mother from changing her Will at all.The terms of your Father's Will will show whether it is a mutual Will or not
The problem is that there is some doubt about whether such a Will can be changed or not after the death of the partner. Some cases say ,yes, some,no. However the court seems quick to impose what is called a constructive trust to prevent the change in terms.A trust is where property is basically held by the survivor for the benefit of the deceased and must be disposed of in the way that the deceased wanted. So again this would prevent a change in the terms of the stepmother's Will.
Finally even assuming the Will is changed then the fact that your stepmother uis confused means that she may not have the necessary mental capacity in law to change the Will and because of the pressure being applied "undue influence" can be argued to have been applied by the daughters.
So the botXXXXX XXXXXne is that you have plenty of legal ammunition to fight against any change in the Will's terms.
I hope this helps. If anything is not clear or there are further points , please reply