If there are surviving children or direct descendants of the decedent, the surviving spouse takes one-third of the personal property in the estate, with the balance going to the children and descendants. The surviving spouse is also entitled to an interest in one-third of the land in the estate for the rest of his or her life (a.k.a., a life estate), with the remainder going to the decedent's children and descendants.
Therefore, filing probate would mean that your mother will receive a 1/3rd interest in the step-father's estate. That doesn't mean 1/3 of allproperty but only that portion of jointly held property that HE owned.
For example, if they owned a home together, she is 1/2 owner by title, then she receives another 1/3 of his portion of the 1/2 ownership and the children receive the other 2/3s