I'm glad you've found the reason, it makes it much easier to deal with when you understand why they are behaving that way. Does your DH spend lots of time doing things with your DD now? I wonder if she would benefit from special "dates" with him, like trips to the park etc. before the baby is here? He probably already does that, but maybe if it was something like a regular scheduled "date" that continues on when the baby is here, she will soon work out that things will stay the same with her daddy?

Hopefully you get to see a child psycologist, they may give you some strategies that you hadn't thought of that could help too.

Best of luck