With both parents dead who inherits the house?

You need to examine the deed to the property. How was the property held by your mother and stepfather. Was the property held in trust? Was it held by your mother and father as tenants in common or joint tenancy or tenants by the entirety? You should have an attorney look at the deed and explain. If held in joint tenancy by your mother and stepfather, then upon your mother's death the entire property was passed to your stepfather as the sole surviving joint tenant. Upon his death it would pass to his heirs. If your stepfather had a will then the will controls who receives the property. If there is no will then the property would pass to your stepfathers descendants. The property would be divided equally among his children, those born to or adopted by him. A probate would have to be opened to transfer the property to his children after all claims were paid. If the property were held as tenants by the entirety then the property follows the same path as in joint tenancy. If the property was owned by your mother and stepfather as tenants in common and there is no will, then your mother's descendants share in one half of the ownership and your stepfather's descendants share in the other half of the ownership. A probate would have to be opened to transfer the property. If the property were held in trust then the terms of the trust would control. There would not have to be a probate.

There are too many unanswered questions. How did mom and dad hold title? If it was in joint tenancy by the entirety then on mom's death dad became the owner. If dad had a will that controls. If dad did not have a will then his kid(s) own the house. Step kids get nothing.