I've actually done a lot of research into this.
Christian's right - a man can be a germline carrier the same way that a woman can. So a man would pass on a bad x via sperm only to a baby that would then be a female carrier. That still doesn't explain Rhainnon's situation, though. I'd be really curious about whether the mutations differed.
Tanya - if you're the first carrier in your family, I would bet money that your mom is a germline carrier. So there aren't any other carriers in the family because your siblings came from eggs without the mutation.
I would caution anyone to put too much faith into the statistics that we're quoted by the geneticists. I now know of three people that have done IVF with PGD, including myself. Two of us are germline carriers, and had 15% and 75% of eggs affected. One person I know if is a true carrier, and she had 67% of her eggs affected. So that 50/50 can be much higher. I think the bottom line is that no one can really tell us what our risk of additional affected children is.