God the Almighty classifies the different categories of women who are mahrams to men in the Quran:
“You are forbidden to take as wives your mothers, daughters, sisters, paternal and maternal aunts, the daughters of brothers and daughters of sisters, your milk-mothers and milk-sisters, your wives’ mothers, the stepdaughters in your care– those born of women with whom you have consummated marriage, if you have not consummated the marriage, then you will not be blamed– wives of your begotten sons, two sisters simultaneously– with the exception of what is past: God is most forgiving and merciful” (4:23).
This noble verse illustrates the categories of women who are prohibited for marriage due to lineage, marital bonds or nursing. According to the verse, some women are permanently prohibited for marriage while others are temporarily so.
The inquirer in this case is classified in the light of the above-mentioned verse as a permanent mahram to the husband of her late daughter through the marital bond. The Prophet (peace and blessings be upon him) said, “If a man conducts a marriage contract with a woman, her mother becomes permanently unlawful for him to marry”—i.e. merely by virtue of the marriage contract.
The majority of scholars and jurists from the four schools of jurisprudence agreed that after conducting a marriage contract, the wife’s mother becomes permanently prohibited for marriage, whether or not the marriage has been consummated.
Based on this and in reference to the question, it is permissible for the inquirer to stay with her son-in-law in the same house. In this case, he is considered a mahram who is still allowed to take care of his elderly mother-in-law and attend to her needs. She is allowed to do without hijab in his presence, travel with him and behave the same way she would with her own son.
God the Almighty knows best.