Is it permissible for a woman to advise her female relatives to wear the hijab in front of their wicked foster brothers?

A brother through suckling is considered a mahram, whether he is righteous, corrupt, or an unbeliever. However, if there is suspicion regarding one of the mahrams, he should be treated as a stranger. Thus, a woman should not uncover before him what she uncovers for mahrams, nor should she be alone with him, nor travel with him.