ANSWER:

No, he’s not a mahram and she must be fully covered in front of him and it’s not permissible to chit-chat or socialize or hang out when he’s around. This ruling is same for both sister’s husband and husband’s brother. 

https://islamqa.info/en/answers/5538/who-are-the-mahrams-in-front-of-whom-a-woman-can-uncover

A sister wants to know if her sister’s husband is her mahram?

Fiqh of the family, Mahram | January 14, 2020 6:23 am
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Sheik my question is that is it permissible for a son or daughter to touch his/her mother’s feet. As many people believe that paradise lies under the feet of mum. And touching them would shower Allahs blessing on you.what is the ruling on it?
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Yesterday my neighbour visited me she said that her daughters proposal and marriage were getting cancelled which is about to fixed..now her daughter is 31 years old. My neighbour is so so sad and since last 4 years she is so sick with this daughter marriage tension.. kindly sir suggest and help in this regard..