Is it permissible for a wife who covers her face to uncover it in front of her sister's husband or her husband's brothers? And is the hadith "O Asma, verily when a woman reaches puberty, nothing should be seen of her except this," and he pointed to his face and hands, authentic?

A woman is not permitted to uncover her face in front of a non-mahram man, even if he is her sister's husband or her husband's sister's husband. The Hadith of Asma, which is used to argue for the permissibility of uncovering the face, is weak (mursal) and is not authentically narrated from the Prophet, peace and blessings be upon him.