Is my father's step uncle's wife my mahram?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Your father’s uncle’s wife is not forbidden to you if there is no reason for the prohibition (of marriage).

Allah Says about the women who are permanently forbidden to marry (what means): {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.} [Quran 4:23]

Also, the Prophet said: “‘Breast-feeding makes unlawful (for marriage) the same thing that blood ties make unlawful.” [Al-Bukhari and Muslim]

Allah Knows best.