ANSWER:

Yes, the father in law is the permanent mahram of the daughter in law even if she’s divorced from his son or his son dies. No need to wear hijab in front of him. 

https://islamqa.info/en/5538

Is Father in Law a Mahram to daughter in law, in whose presence she doesn’t have to wear Hijab?

Mahram | October 11, 2018 5:57 am
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Sir my question is that my sister s husband is my mehram? because someone says there is no need of face covering before him.please guid me.
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I am a female planning to go for umrah with my son and want to take my niece along with me. Can my son (18 years old) be a mahram to my 27 years old niece (my sisters daughter)?