My husband is divorced with children from first wife. I am the second wife, no children. Is my husband daughter's husband my Mahram?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Your husband’s daughter’s husband is neither among the permanent Maharim of you nor the temporary Maharim.

Therefore, you should not uncover yourself in front of him, should not stay in seclusion with him, should not shake hand with him and should not travel with him if not accompanied by one of your Maharim.

Allah knows best.