Is it obligatory to wear the hijab in front of the husband's sister's son (nephew) who is 12 or 13 years old?

A Muslim woman must observe hijab in front of her husband's nephew if he is an adult or near adulthood, because he is not a mahram, unless he is a mahram for another reason such as suckling. The husband's relatives are considered non-mahrams to his wife. The Prophet, peace and blessings be upon him, warned against the hamw (husband's brother or relative) by saying: "The hamw is death."