Without a Will, your father's estate should be equally divided among his rightful heirs; in this case that would be you and your brother. So any assest of the estate is 50% yours. That having been said, a 401k is not consuidered to be an estate asset and passes outside of probate. Only the designated beneficiary of the 401k has rights to it. If no beneficary was designated, then it will go into your father's estate. Bottom line, your brother may have rights to the 401k if he was the only name listed as the beneficiary. At this point, you should consult directly with an attorney in your area.