Thanks all! I called my sister and asked that she let my mother know that she told me. We chatted a bit - my mother thinks he just gets upset and says the first thing that comes to mind.
They’ve been under a tremendous amount of stress lately (job loss, death of my step-dad’s mother, etc) , and things have reached a boiling point. Relations between my step-dad and sister have been emotionally contentious although they both care a lot about each other.
More importantly, my mother has given me her blessings to communicate with my step-dad about his relationship with my sister. This has caused added stress and my mother is “up to here” about it, which is making her a bit short-tempered as well. I truly think that they (sis and step-dad) are both to blame and both are blameless. It comes down to a communication problem. Having gone through cognitive behavioral therapy, it’s pretty easy for me to spot the flaws in my own thinking, and somewhat in others too.