This is a very common and permissible question that people ask. Is son in law a mahram for Umrah? Or is son in law mahram for Hajj? The answer is yes. It is clearly stated in Surah Nisa verse 23:
Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives’ mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.
Here it clearly stated that your wife mother is prohibited for marriage. Hence, it is clear that your mother in law is someone you cannot marry. With that the son in law automatically becomes a marham.
A mother in law is just like a mother. She has given her daughter to the young man. Performing Umrah with extended children is exceptional act. And the son in law considers the mother of his wife like a mother too.
Do remember that if you take your mother in law then you need to have obligations like a mother. The mahram is responsible for the wellness. He has to take care of the mother in law just like a mother. She has the rights of a mother. So be very particular in providing goodness.