Legal Question in Civil Litigation in Oregon

Your question is not clear. Who is your step family? Why would your father leave his estate to your step family?

In any event, you can hire an attorney and contest the will as a "pretermitted heir" (child who is not mentioned in the will). But that does not work, if your name actually appears in the will and is expressly left something or expressly left nothing. In those cases, it is very difficult to get a will to be countermanded by the court.

You could also charge that the step family somehow forged the will or caused your father to sign a new will after he had lost mental capacity. But you would need to prove the facts of those allegations.