The maternal and paternal uncles of the husband are not mahrams to the wife

No; the paternal
uncle and maternal uncle of the husband are not mahrams; rather she cannot
uncover in front of the husband’s paternal uncle and maternal uncle, or
the husband’s brother who is closer to him. Rather the words of Allah, may
He be glorified and exalted), “and
not to reveal their adornment except to their husbands, their fathers”
[an-Noor 24:31]
refer to fathers and grandfathers, which does not include paternal uncles
or maternal uncles. 

The one who says
that they are included is mistaken; rather the fathers are the fathers and
grandfathers, no matter how far the line of ascent reaches. As for the
husband’s paternal uncle, maternal uncle and brother, they are “strangers”
and are not mahrams, so she cannot uncover in front of them; rather she must
observe hijab in front of them. Neither the maternal uncle, paternal uncle
or brother of the husband are mahram to her. Rather the only ones who are
her mahrams are the husband’s father and grandfather, the husband’s son, and
the son of the husband’s son or daughter (i.e., his grandsons). The
husband’s father and his paternal and maternal grandfathers are mahrams, as
are the husband’s sons and the sons of his sons and daughters. All of them
are mahrams to his wife. End quote.

Shaykh ‘Abd
al-‘Azeez ibn Baaz (may Allah have mercy on him).