Hannah Lindley, 20, from Leeds, said a health advisory body's recommendation means she will suffer the same fate as her sister.
Iona, 19, died from the condition in November last year.
The draft guidance for the Orkambi drug says it is not a "cost effective use of NHS resources."
In trials the drug, which costs Â£104,000 a year per patient, reduced instances where users experienced a sudden worsening of symptoms which required hospital treatment, but the National Institute for Health and Care Excellence (NICE) said the benefits were modest.
"With CF you can be OK one minute and you can have a chest infection and be dead a month later, so I don't know how long that will take," Ms Lindley said.
"I'm going to have a life of worry and sickness, basically.
"Not having this drug will mean I will have the same fate as my sister, which is deterioration until ultimate death and why should I be given a death sentence for something I did nothing to cause?"
James Barrow of the Cystic Fibrosis Trust said: "We think it's wholly wrong that people with a life-shortening condition are caught up in a cross fire between a pharmaceutical company and the NHS over long-term effectiveness and data. So we are saying why not give the treatment to people with the condition and then measure the impact it's having while they are receiving it?"
Prof Carole Longson, director of the NICE Centre for Health Technology Evaluation, said it was "disappointing" it was not able to recommend Orkambi.
"When compared to the current standard of care, the benefit it offered was modest and comes at a considerable cost," she said.
The draft guidelines will now go out to public consultation.
A cystic fibrosis sufferer has said the decision not to provide a new drug on the NHS has left her facing "a death sentence".