First of all, we’d like to state that the Islamic rules of covering for women are intended to safeguard and preserve their honor and dignity. Rules of covering vary according to whether the degree of risk of temptation is greater or lesser; where such risks of temptation are greater, rules of covering are stricter, and where the risk is minimal, rules are minimal.

Rules of covering are, therefore, stricter in the presence of males who are non-mahram (who are not related to the woman by blood, marriage or fosterage). If this uncle is your mother’s or father’s brother he is mahram to you and you may remove your hijab in his presence. We would like to add as clarification, however, that a woman should wear very short, tight, or low-cut clothing only in front of her husband. In front of other mahram men her clothing should be more modest, but she does not have to cover completely as in front of non-mahram men.

Shedding more light on your question, we’d like to cite for you the following:

“A non-Muslim relative may be a mahram or a non-mahram. As for males who are considered mahrams (those who are related to the woman through marriage or through blood or fosterage and whom she can never marry) like father, brother, uncle, etc., a Muslim woman is allowed to display her adornment before them even if they are disbelievers because of the generality of Almighty Allah’s saying: “… and not to reveal their adornment save to their own husbands or fathers or husbands’ fathers, or their sons or their husbands’ sons, or their brothers or their brothers’ sons or sisters’ sons, or their women …”
(An-Nur :31). However, if those mahrams are immoral, she is not allowed to take off her hijab or display her adornment before them because the licentious do not differentiate between their mahrams and non-mahrams.

Also, she is not allowed to display her adornment or take off her hijab before her non-mahram relatives.”