We are in a similar situation and it's caused issues.
I've been told the same as you so I'm being shielded.
I'm germ phobic at the best of times so my attention to risk control is very good. I also had infection control training decades ago.
Having anyone coming and going in your house is a no. Under any circumstances.
The end.
One solution might be you isolating in part of the house whilst your husband and his daughter have two weeks quarantine in another part.
You'll need a physical barrier to divide the house.
You cannot be in contact with any part of his side of the segregated house.
Meals can be delivered to a safe area outside and collected. Plates need to be put in a submersible container containing a warm bleach solution by them, before being handled by you (you'll need to be doing the food prep as it's safer).
After the two weeks once you know she's not a risk, your step daughter stays for a third week so you can see her too. Then returns to her mum.
If you only have one bathroom it's going to be impossible to do this with an effective barrier though.
The other way is he moves out and has her to stay somewhere away from the house. He will need two weeks minimum to make sure he's not bringing anything back to you. But during the last two weeks of this time away he needs to be socially isolating to protect you on his return.
We bought a caravan for this scenario. I saw it coming and thought through measures to stay alive. We have just had a child return from university where there were cases, so it's proved essential already.
I'm so sorry you are going through this, it's not that you don't love all your children. It's just unprecedented times and staying alive with chronic lung conditions is a top priority.
I've felt like a really horrible person, but trying to explain high levels of barrier infection control to a lay person is so so hard. Especially when missing children is in the mix.
If you died with your unborn baby he would be eaten up with grief and guilt. And you'd not be there to be a mother to all your children.
You are doing the right thing. Absolutely. But work out options and let him have a choice.