You are right to balk at changing your mother’s will to give you a bigger share. That would leave the will open to a legal challenge from your sisters, who
could claim that you exercised undue influence over her when making this change.
Your mother can make changes to her will as long as she has the legal capacity to do it. In the eyes of the law, this means she must understand the nature, scope and effect of the document—and be able to remember and understand who the beneficiaries are and what property they will get.
The capacity requirement is measured at the time the document is made or changed -- and that can be a tad tricky. Some people with Alzheimer's, for example, intermittently have the mental capacity that is legally required.
If your mother wishes to change her will and has the legal capacity to do so, given the possible distress this could cause in the family, the change should be made by an attorney and must be witnessed by at least two people who could later attest to your mother’s capacity if questioned.
Many attorneys who focus on elder law will be willing to come to a residence—the legal equivalent of a house call—if that would be most convenient for your mother.