What should I do about the deviant behaviors I observe in my sisters' children, given the absence of a supportive man in the household?

Raising children is a duty upon parents, and Allah will question them about this trust. The people most responsible for their care are their living fathers, and it is incumbent upon them to strengthen their children's faith, monitor their behavior, prevent them from associating with bad companions and using corrupting means, provide them with righteous company, seek help from others when incapable, and not abandon supplication.