Whilst checking what the other posters have said, I would also be looking at the attention aspect. She has had a lot of upheaval in her life and this could be her way of dealing with that.. Also I notice with my DD, a few days older than your DD1, that if we have had a bad run of things where she has not had enough Mummy time, she has 'accidents' for attention (I know some a genuine accidents, but I also know some aren't, not just toileting, general things). No matter how much I share my time with them individually and together, they only seem to see that the other is getting time, and forget about the last hr that was focused on them Is it possible your DD1 sees that nappy change time with your DD2 is 'fun' and she wants to go back to that safe and secure time with you?

Hope you get some answers and can help her move on from this..