If my father-in-law is married to a woman other than my wife’s mother, is she considered a Mahram to me?

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

The wife of your father-in-law is not a Mahram (non-marriageable relative) to you, merely by being your father-in-law’s wife, as she is not your wife's mother. You are permitted under the Sharee‘ah to marry her, even while still being married to your present wife, as long as there is no other impediment under Sharee‘ah, as in the case when she is your wife’s maternal aunt.

Allaah Knows best.