Sibling Rivalry

I have worried for a while about how Ella will cope with the change in our family dynamic when baby arrives. She has been back and forth between really excited, wanting to be the grown up big sister and being less interested, almost regressing back to baby-hood herself.
I have thought all along that Lee is going to have a harder time of it than me but he has always disagreed. I am her Mum after all so he assumes she "loves me more" but he doesn't understand. To Ella she's never known anything other than a Mum and Dad and a Stepdad. He's as much her parent as I am. If anything it will be worse for her with him when baby comes along because he's her Stepdad. I don't think it's about her preferences I think she's a smart kid and right now she knows she's the closest thing he's got to a child but once the baby comes along that wont be the case anymore. If you were Ella would you not feel threatened by the arrival of a biological child? I liken it to a family that has adopted and goes on to have a child of their own, that adopted child is going to feel a completely different kind of jealousy and resentment.
Ella knows I am her Mum ans I am going to be Milo's Mum too but Lee is going to be this baby's real Dad and even at 6 she knows there's a difference.
I really don't know how to reassure her without making a big issue of it but have suggested that we each have an activity we do with Ella alone and then perhaps a film night once a week when the baby is in bed and she can stay up late. It's a very tricky situation for any family but possibly more complicated still for a "blended" family such as ours. I'm sure we will manage but I wanted to share my thoughts in the hope it would open a discussion among you who have an opinion or have even experienced a similar situation.
This evening Lee told me he finally believes I could be right about her jealousy focusing on him. Ella has always been very affectionate towards him and to anyone watching them together you can see how much she loves him but she's never been overly vocal about her feelings to him. Apparently while I was out of the room today and they were cuddling on the sofa, as they do, she turned to him and said "I like you". Given the amount of time we have been a family unit it may seem like a strange thing for her to say but I think the timing tells us she just wants to make sure he knows.
I've heard her say she loves him before but smart as she is we always suspect she is after something, on this occasion there were no follow up requests, just a spur of the moment expression of fondness.
I hope the solo to sibling transition is smooth. I know that my daughter is loving and kind natured, a big part of me thinks I have nothing to worry about, she will be a big help to us and the perfect big sister but she is still my baby and I want to do everything I can to ensure she feels loved and part of our new family.

Lee wrote a post about Step-parenting, you can see it here