1. Is it permissible for the paternal uncle's wife to uncover herself before her husband's brother's sons, who are considered her mahram? 2. Do the older and younger siblings of an infant who suckled from a woman become children of the nursing woman, or is this limited to the suckling infant only?

The wife of the husband's brother is considered a stranger (non-mahram) to her husband's brother's sons. Therefore, she must wear the hijab in their presence, and it is forbidden for her to be in seclusion with them. An exception to this is if one of them becomes her mahram through fosterage (رضاعة). In such a case, he is permitted to see what a mahram can see, such as her hair, neck, palms, and feet, provided there is no fear of temptation (fitna). Seclusion and shaking hands are also permissible. Fosterage establishes prohibition just like lineage by birth. So, whoever is nursed by a woman becomes her son and the son of her husband (the owner of the milk). This applies only to the foster-child himself, not to his siblings.