Hello Joe3825, glad you found us and

Confusion is certainly the right word for your situation. From our perspective, the behavior of a pwBPD makes no sense; it's irrational. If she thinks you're "bad" right now, wouldn't a good mom keep her child away from a "bad" person?
That kind of disconnect is more of a feature than a bug of BPD. BPD is a real, serious, and impairing mental illness that affects the sufferer's beliefs about the world (and themselves), perceptions of reality (hypersensitive in some areas, underreactive in others), and emotions and regulation.
Many pwBPD also cope with "black and white" thinking, which could also be described as "all or nothing" thinking. Grey areas can be difficult for them, so often they seem to be un-integrated in their thoughts. I wonder if in her "partner" view of you you're all bad, but in her "stepdad" view of you you're all good. A "generally normal" person might be able to integrate those but she is pretty impaired so she's doing things that don't make sense to us... but make sense to her.
Given that you're here on the bettering board, it sounds like you want to stay in, and improve, the relationship?
If so, a good place to start is our section of articles on
When a partner, spouse or girlfriend has Borderline Personality Disorder. Not much about a BPD relationship is intuitive, so it takes learning a lot of new, often unintuitive tools, skills, and approaches to make things work. Take a look when you have a chance and let us know what stood out to you.