Have you talked to them about how they feel about their step-mom? About their situation with their step-mom and dad? If you're angry, hurting, and emotionally drained, maybe they are too. They may be reacting to stress that's going on in their lives, especially if they're emotionally affected by their stepmom's serious health problems, which I'm sure they are. Maybe try sitting down and having a calm talk with them about these issues, and listen to what they have to say. They may fight constantly, but the root cause could be about something completely different, like their parents.