All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Your wife`s stepmother isn`t among your Mahrams. Rather, she is a non-Mahram with whom you aren't allowed to shake hands or stay in private or look at other than her hands and face. The evidence on this is the verse which specified the women forbidden in marriage including the wife`s mother, but didn`t mention her (Wife`s) stepmother. Allah the Almighty Says {What means}: "Prohibited to you (For marriage) are:- Your mothers, daughters, sisters; father’s sisters, Mother’s sisters; brother’s daughters, sister’s daughters; foster-mothers (Who gave you suck), foster-sisters; your wives’ mothers; your step-daughters under your guardianship, born of your wives to whom ye have gone in,- no prohibition if ye have not gone in;- (Those who have been) wives of your sons proceeding from your loins; and two sisters in wedlock at one and the same time, except for what is past; for God is Oft-forgiving, Most Merciful." {An-Nisa`, 23}. And Allah The Almighty Knows Best.