I wouldn't say you're at "fault", but losing your temper was a mistake. It sounds to me like these people are a malignant presence in your life. I wonder why your sister wants you to help your step-father. Is she his daughter? Anyway, it would be best to find a way to diffuse the situation and cut all these people out of your life if you can. If these kinds of interactions persist, maybe recording it somehow would be a good idea(I'd do a bit of research first). There's no reason for you to feel bad based on what you've written.