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