Here's some ideas for phrasing. Any thoughts are helpful.

She has sustained severe injuries. I cannot say how long she might survive. We are preparing to deliver the babies. She's carrying twins.

Her injuries are severe, but she is fighting for survival. I cannot say how long she might last. We're preparing to deliver the babies.

Her injuries are rather severe. She is stable, but I cannot say how long she might last. We're preparing to deliver the babies.

Her injuries are severe. I cannot say how long she might last, but her determination is evident. We're preparing to deliver the babies.

Despite her severe injuries, she has a strong will to survive. I cannot say how long she might last. We are preparing to deliver the babies.

She has a strong will to live, despite her severe injuries. I bet she'd live long enough for at least one of the babies to remember her.