I know this probably won't help but your mom is probably saying those horrible things out of fear and anger. She is probably attempting to defend herself by taking you down. It is possible but i think it is unlikely that she has really felt that way about you for the past 8 years. You have every right to be angry though. Under no circumstances should a mother say those things about her childern. Just because she is afraid and angry does not justify her taking it out on you. It must be so hurtful to hear those things though. I am so sorry. Maybe someday she will understand that you were only acting out of love and concern for your brother and sister and that you never meant to attack her but i imagine it will be very long time before/if she ever realizes that.
It sounds like your mother is incredibly frightened by addiction and that she is afraid your brother and sister will become addicted to medications. Maybe there is someone, a doctor or a soical worker, who she could talk with who would validate that fear for her but explain that it is not an issue with the medications that CFers use. They are not addictive drugs. maybe they could explain that treatments might cause more coughing but that increased coughing is sometimes a good thing (because it means mucus is being cleared) and then she won't feel so afraid of the medications and worry that they are harming her childern. I really don't know but it seems like someone who is educated about CF has to be able to talk to her! I think they will have a better chance at getting through if they are compassionate and try to address her fears!
This sounds so incredibly difficult and sad for you. Please try not to be too hurt by what your mother says about you. You are a wonderful, loyal daughter and sister.