It is difficult to watch the abuse and deception of a loved one, especially when you feel powerless to prevent it. Unfortunately, a bit more information is needed before your question can be answered. Your father is 90, but age is not an indicator of incapacity by itself. Does your father have any medical conditions or frailties that make him vulnerable to undue influence by your step mom? How long have the two been married? Does your stepmother have some manner of legal control over your father (i.e. is she his conservator or is she a designated attorney-in-fact). Also, you would need to clarify what forms of abuse he is a victim of.
There are statutes in place that are designed to protect people like your father but more information is needed to determine if he is truly a victim of abuse and/or undue influence.