Yes, that scenario would be if your father is not in fact your biological father and the man who is, is also a carrier and passed on his CF gene to you along with your mother.
My oldest daughter had her gene typed and the girls' docs said it was not neccessary to type Rachel at the time. They said later on before she came off our insurance we would want to have her typed so in the future if an meds. came through the pipeline that were gene specific we would need to know what her mutations were. Assuming that they both have the same parents then their mutations would be the same. Well I knew that to be a fact so we didn't have her typed then. About three years later we asked her new CF doc to type her and he said it wasn't neccessary.
After talking with the CF coordinator she explained he was usually hesitant to have patients mutations typed (unless it was needed for some reason) because he had come across a number of patients that ended up revealing the father's of record were not really the patients biological father's. I told her that I knew for a fact who her dad was and I had nothing to hide, one, and two, with PTC 124 coming down the line I wanted it documented that she had a nonsense mutation. So she was typed and sure enough it came up the same.
You can be reasonably sure that you and your sister will have the same mutations if you have the same parents. You should get typed when you can get the test covered so it can be documented. I think you can make a point with the letter from the doc. if your sisters mutations come up as nonsense mutations because there are meds coming down the pipeline, that if success continues, are based on whether you have a nonsense mutation or not (PTC 124). I believe there is also one being researched for Df508, but I can't be for certain what that one is as my girls have both nonsense mutations.
Good luck with getting your test covered.
