Stefan, I think ryanbutler is saying that the tab order is not even going into the radio button group, but skipping right past it. I have tried aligning the hierarchy to match the tab order, and had a little more success.
Otherwise, just use checkboxes, there much easier to use for creating the forms, and for the end users. Most end users don't know how to use the Tab/Arrow/Spacebar keys to select the radio buttons anyway. At least with check boxes, users just need the Tab/Spacebar keys.