Am I allowed to contact health professionals on her behalf? Isn't it confidential? X
Do you know who her health professionals are?
You cannot contact her health professionals and speak on behalf of your sister not her unborn child. If you tried to do this, a health professional who is behaving in a professional manner would not take you as having spoken for your sister or her unborn child in any case.
What you can do is contact your sister's health professional, make it clear who you are and who your sister is - have her name, address and date of birth to hand, perhaps, to ensure that the information you share is connected to the correct patient - and inform the health professionals of the concerns you have raised.
They may or may not take any action, but this is something that you can do. If your sister finds out that you have done this, there may or may not be fallout for you and your relationship with her, so you might want to think carefully before taking any action.
My HCP have been contacted on my behalf a number of times when friends or relatives had concerns about me.
The HCP should not give you information about your sister nor her baby without your sister's specific consent. That is where the confidentiality comes into it.
I am not saying that you ought to do this. I am explaining what it is that you can and cannot do.