Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
You cannot marry your mother-in-law at all. She is an everlasting mahram to you.
Answering your question, Sheikh Kifah Mustapha, the Imam and Director of The Prayer Center of Orland Park and representative for Dar El Fatwa of Lebanon in the US, states:
You cannot marry your mother-in-law at all. She is an everlasting mahram to you. Even if you just concluded the marriage contract without actual consummating the marriage, your mother in law is your mahram forever.
Allah said: {It is prohibited on you to marry your mothers, daughters, sisters, aunts, nieces, mothers who nursed you, sisters in nursing and the mothers of your wives} (An-Nisaa 4:23)
Read also:
What Are the Conditions of Marriage in Islam?
Is Secret Marriage Allowed In Islam?
What Invalidates Marriage in Islam
Does Islam Condone Mutah Marriage?
Almighty Allah knows best.