Legal Question in Wills and Trusts in Missouri

Virtually anyone can apply to open a probate estate. Whether or not you will appointed depends on a variety of factors, primarily whether or not someone objects. If they object then a Court will determine who should be appointed. Missouri statutes do provide a "pecking order" of who will be appointed in the case of a conflict.

You will need an attorney to open a probate estate in Missouri

If there is no Will naming an executor, as to your mother's estate either you or your brother would be given preference under MO law to be appointed personal representative of her estate. As to your stepfather's estate his children and/or other blood relatives would be given preference. Unless he owned assets that are to pass to you and/or your brother there would be little reason to go through the work of handling his estate (that is unless he has no one else to do so). Best of luck.