Alex is at the stage where she is napping some days and not on others so I do think that Elodie might be due to drop it soon. I certainly think that she shoudl have it as early as possible and try to cut it down to 1.15hrs. and no later than 2.30pm wakeup.
I do also think that it is down to a little SA due to the expected arrival of the baby. No doubt you have told her that she is due to have a little brother and I am sure that, along with excitement, she is feeling rather nervous about it. Understandably so.
I agree that she should be old enough to understand things if you explain it to her, and you should tell her that mummy is tired and she must go to sleep. A reward chart might help at this age, I did one for Nathan, but he was four at the time. Keep it simple, explain what you want her to do and let her put the stars on in the morning, she might get to be excited about it.
I really don't think that closing the door and keeping her in her room is a good idea though, I think that it could cause more trouble, especially at this crucial time, this is the time that you need to really reassure her, that is what she is looking for.
You should always be there and let her know that you will answer her needs and do whatever you can for her, but also be firm that she must go to sleep. maybe say something like 'Go to sleep, we're tired, Elodie is tired and so is mummy, now is time to sleep.' Then, when she wakes later, do less for her, jsut put her back to bed and leave.
I hope this helps a little, but TBH, Nathan was apprehensive before Alex was born but after she arrived he was a lot more settled about things. I let him get involved as much as possible (he even bathed her, that was fun!
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Mari