Okay so maybe I'm dense but I couldn't figure out how to get mink to just select a radio button based on its label value. So for anyone else trying to figure it out here's a step you can use:

     * @Given /^I select the "([^"]*)" radio button$/
    public function iSelectTheRadioButton($radio_label) {
      $radio_button = $this->getSession()->getPage()->findField($radio_label);
      if (null === $radio_button) {
        throw new ElementNotFoundException(
          $this->getSession(), 'form field', 'id|name|label|value', $field
      $value = $radio_button->getAttribute('value');
      $this->fillField($radio_label, $value);