Is it permissible for me not to wear a hijab in front of my husband's daughter's husband, or not?

Your husband's daughter's husband is not a mahram to you because there is no basis for mahramiyyah. You must observe hijab in front of him, and it is not permissible for you to uncover in his presence or to be alone with him. However, your daughter's husband is a mahram to you, as Allah Almighty says: "and your wives' mothers."