From the same article on wikipedia:
The inbreeding is computed as a percentage of chances for two alleles to be identical by descent.
Now let's draw a family tree for such mating and sign chances for having each allele:
You can see, that the "brother" has a 50% chance of receiving A allele and the "sister" has a 50% chance of receiving A from their mother, but the "brother" will not have the C allele that the sister certainly has. Their daughter will have the allele from her father and one of mother's alleles. Therefore, her chances of having a C allele are 50%, and for a A or a B allele each 75% (remember, she needs two copies, so we need to end up with 200%; furthermore, she may have two A alleles but not more than one C allele). Chances of her having two A or two B alleles are 56,25% $$0.75 * 0.75 = 0.5625$$
Now we have the percentage of chances for two alleles to be identical by descent, her inbreeding coefficient is 56,25%.
The son will have only one allele, 50% for C, 25% for A and 25% for B. Because he does not have two alleles, by definition there is no inbreeding possible.