So, it looks like she's going to go ahead and have the baby, so I suppose it's time for your DH, your DSD and you to work out the practicalities. I was thinking about how it might fit around your DSD's schooling as she comes up to GCSEs.
Is the baby due round about March - April? If so, can she leave school for the year in March, spend April - August looking after the baby full time at home, and go back to school to take her GCSEs for the next academic year, leaving the baby in full-time childcare from about 5 months? So that's a nursery for 5 days a week, which your DH will need to budget for. (I wondered if you'd need to look at remortgaging the house to pay for this?) If your DSD takes a packed lunch to school (maybe prepares all her sandwiches at the beginning of the week and freezes them), she could do some of her homework at lunchbreak in the library, then another hour of homework in the library after school, and then come home and is ready to take full care of the baby during the course of the evening, and in the morning before she leaves for school. Then Saturday and Sunday she's got the baby all day. It might mean that she needs to choose GCSEs that have coursework she can reasonably do in short blocks of time at school, rather than coursework that need a weekend staring at a river bank and making observations, or trips abroad, etc. Music might also be difficult if there's a performance aspect. So I think consultation with teachers on practical GCSE options in light of childcare may be needed.
Getting the baby to and from nursery may be difficult until your DSD can drive - I wonder whether your DH would do drop offs before work, and you do pick-ups after your work?
You said you're a freelance, which means that your business are entirely dependent on you and your reputation for efficiency alone, and your job is far more at risk than your husbands (who presumably works as part of a large team of people) if you're taking time off to look after the baby. I think that you should start working out of the house (renting an office elsewhere), and on fixed days at fixed times from now on, to make it clear that you are not available to do back up childcare. If not, I think your business will be the one that suffers. Do you have a office in the house at the moment? You can use the need for a bedroom for the baby as an excuse to stop using the office.
Hope that's of some vague help.