I agree with Frontal Bossing.

The parents of the mother have to be carrier's since her sister has the disease but they do now show the phenotype (CF is auto recessive)

Mothers parents are both Aa

Therefore, the mother's potential phenotypes are: 1/4 AA, 2/4 Aa, or 1/4 aa but since she does not have CF, she cannot be aa, leaving 3 possibilities. She has a 1/3 chance to be AA and 2/3 to be Aa based on probabilities.

The mother has a 50% chance of passing on the a allele if she is a carrier, so overall, her chance of passing on the a (recessive) allele is 2/3 x 1/2 = 1/3

The father has 100% chance of passing on CF gene, therefore the answer should be 1/3

Hope that made sense/is correct :sorry: