For what you're trying to do, forget P, Av or Tv...try (M)anual mode. Put the camera on a tripod, set the aperture where you need it (e.g. f/11), set the shutter speed to allow some ambient light in (e.g. 1/15) and let the flash provide fill (I would recommend bouncing the flash for best results).
You might even try bumping the ISO up from 100 to 200, or 400.
If you want to use the smaller aperture settings, you have to use Av mode or manual. You should use a tripod, because you are going to have a long exposure. Any ambient lighting turned on in the house (fluorescent, incandescent) will cause an imbalance with your white balance.
If your flash is set for ETTL, it should give you a working range for the exposure. It is possible that your flash in underpowered for the small aperture, in which case you should consider a more realistic aperture setting (5.6-8).
