What is the ruling on interaction and hijab in front of the the paternal aunt's husband's brothers?

The basic principle is that the stepmother's brothers are non-mahram (strangers), so private seclusion with them is forbidden, and full hijab is obligatory in their presence. This is unless they become mahram for a specific reason, such as prohibitive suckling, in which case they are permitted to see what a man may see of his mahram women (such as the head, neck, hands, and feet).