Is the wife required to wear the niqab in front of her husband's mahram brothers, especially if they were young at the time of marriage and she considers them like her own brothers?

If the wife's brothers are her mahrams through legitimate suckling, then she is not obligated to wear hijab in front of them. Her awrah with them is her entire body except for the extremities, as long as there is no fear of fitnah. However, if they are not mahrams, then they are non-mahrams (ajaneb), and she must observe full hijab in front of them. It is forbidden for them to look at her or to be in seclusion with her. The Prophet (peace be upon him) warned against men entering upon women, especially the husband's relatives.