Is it obligatory for a woman who is engaged to wear hijab in front of her adolescent mahrams from among her nephews and nieces (children of her brothers and sisters) to avoid her fiancé's jealousy, knowing that she wears short-sleeved clothes in front of them?

The limits of a woman's 'awrah (intimate parts) in front of her mahrams (unmarriageable kin) have been previously explained. However, if there is suspicion or fear of fitnah (temptation/strife), then caution and covering are obligatory. The suitor is a stranger to his fiancée until he marries her. Jealousy over one's family is commendable, as long as it does not exceed the bounds of moderation.