Pregnant women do appear to be at higher risk (although the risk is still low) so imo no, you're not 'being over the top'.
It's difficult because it's your sister's wedding, but I would consider not going. I believe that the two 'hot spots' for swine flu are Birmingham and London, so if guests come from those areas, and you combine that with lots of people in a confined area, touching plates, glasses, toilet door handles, etc. then I reckon there is a reasonable chance you might pick up the virus.
The chances are that you won't catch it, nor pass it on to your wife, and if you did, the chances are that she would be fine. But I doubt that I personally, would take the risk.