It sounds like she has organizational problems and maybe she herself gets overwhelmed by the group aspect. In fact, what you describe sounds a lot like me. I forget what I'm doing and get disorganized so easily. But, you know, I try to set up tricks and things to help me out. She should have been aware two new people were coming and prepared in advance for that.

Anyway, I understand you were upset and it's not your fault. In that situation, being disorganized or not doesn't matter, you still have to try since other people are involved.