What a terrible and happy story!
Personally, I don’t find this a huge moral dilemma. The doctors were not trying to kill Rosie, they were trying to save both babies. Rosie was just in such bad shape she couldn’t survive apart from her sister.
It’s better to lose one life than two.
In ethical terms, the death if Rosie was a completely foreseen, but completely unintended, double effect. If they could have saved both, I am certain they would have done so. An in fact they kept trying to do so, within the limits of the possible.
It occurred to me when I read your comment. God the Father made that exact decision. One life for each of us.