I think those who responded without insight should not have piled in - nobody can put themselves in your shoes unless they have faced similar themselves, no matter their moral standing on broader issues raised.
I never wanted a second child, and then DC1 died. I now have DC2, who is upset there is no sibling to play with- but who is a joy and I am very glad I had him. Not for one second will he ever replace or come near to replacing DC1 and this in itself raises a whole ton of murky ambiguity deep within and has indeed raised issues for him too despite his young age.
I really think it's important to make this decision on the basis that DD will live. You really must think positively - she is in treatment - think positively please.
Secondly only then if you think you will cope with DD sick and a new baby, can you proceed and choose to have that new baby.
Thirdly - it would be particularly bad in my experience to be pregnant while experiencing a bereavement. So if you are intending to only have another child if DD dies - which is understandable but not easy reading for most I'm sure - if tht happens which hopefully it will not - but then my advice to you is to wait, let the dust settle, take in what has happened, grieve, and then decide to have another child, which may lessen pathogical grief reactions and impact on your health and the baby, even delaying your reaction completely.
So, no - what you are thinking makes sense - but whatever you choose now must be with the firm belief DD will survive and you will have 3 children.