Answer
From the information given, and since cystic fibrosis is a recessive trait, for Tom and Beth each to have an affected sibling (let N=normal lung function allele, n=cystic fibrosis allele), all of the parents must be Nn. Thus, it is a 2/3 chance that Beth will be Nn, and the same for Tom. If they are both carriers, any child of theirs would have to be nn (1/4 probability) to have cystic fibrosis. Thus, the chance is 2/3 x 2/3 x 1/4 = 1/9.
Work Step by Step
The key here is to realize that since Tom and Beth do not have cystic fibrosis, they are either NN or Nn, making the probability of Nn 2/3 (do the Punnett square for NnxNn which is the cross which produced both Tom and Beth. Also, if each is Nn, then see this same NnxNn Punnett square to get the probability for a child of theirs having cystic fibrosis.
