What can be done to stop a stepdaughter from taking financial advantage of her stepfather with dementia?

Call the cops. There are many agencies that would love to throw her axx in a penitentiary. If you have hard evidence, see your district attorney.

Need more info. Is this stepdaughter the biological child of this father? Is this your stepdaughter? Is this father your husband.? If this is your husband then you are legal next of kin not his offspring.