As a divorced mom who is about to get remarried to a great guy, good for you! It takes a really kind, patient and devoted man to open his heart and take on the responsibilities of being a dad to a stepchild.
Also- again, as a divorced mom, I have to say, I don't know many 6 year old girls who sleep in the same bed regularly with their fathers. I don't want to alarm you or your wife if there is no reason to be concerned, but at the very least, it sounds like it is contributing to your daughter's fixation with not wanting to sleep in her own bed.
Can you and your wife meet with the ex ( I know it is a pain, but you gotta do it sometimes, right?) and tell him that with the baby coming and the move to the new house, your daughter needs to get comfortable sleeping in her own bed and that having her sleep in the same bed with her dad is not the best thing for HER well-being and adjustment right now. That way you aren't accusing him of anything.
Then offer to get your daughter a special sleeping bag or air mattress or whatever for when she sleeps at the dad's house. Sorry- but at the very least, she could be sleeping on the couch. Where is he planning on having her sleep for the weekend when she is a tween or a teenage girl??
Maybe your wife has also sort of encouraged letting her daughter sleep in bed with her as a comfort mechanism for herself as well as the child. Getting divorced is very difficult- especially if her ex was abusive- and nothing is more comforting than having your daughter safe snuggled up with you!I don't even know if you should bring that possibility up to your wife, but that may be part of why she doesn't want to 'let go' of that habit either.
I would suggest that you both want to encourage your daughter to be independent and brave- and most importantly to feel SAFE in her new bedroom and new house. She is not going to learn to feel that her room is a safe place if every time she wants to leave it, you swoop her away to your room.
But you and your wife need to be on the same page with this, especially with the baby coming. I agree with what most other posters have said- lots of kids want to sleep in mommy and daddy's bed, but it just isn't always the best situation for anyone!
Set up your daughter's bedtime routine and stick to it! Let her pick out a special nightlight for her room, etc. If she gets up and wants to come into bed with you guys, you and your wife need to gently but firmly tuck her back in. Over and over again, as many times as it takes. There will be some crying and fussing at first, but eventually she will just get used to it.
Remember, you are going to be exhausted and up all hours with a new baby soon- everyone in the house is going to need all the sleep they can get, so better to get your daughter settled into her sleep routine now. Play up to your daughter about how lucky she is to be the 'big girl' and have her own room, bed, princess bedspread, etc.
There will probably be some regression when the baby is born- that is PERFECTLY normal and happens with all kids, so don't let it throw you! There is going to be some drama- little girls are drama queens, lol, but just try not to let it get to you- it is all normal and she will outgrow wanting to sleep in your bed if you just stay calm, consistent and upbeat about 'her' bed in her own room.
Good luck-= I am sure it will all work out! Congratulations on your new house and the new addition to your family!!