What will alcohol do to you as a diabetic? It depends. Here's the simplest explanation I could come up with ... if you drink beer, fortified wines (like port) or non-diet mixers with spirits, and certain other drinks like alcoholic cider, your BG will initially soar.
However, as your liver cuts in to deal with the alcohol you've consumed, your supplies of glycogen (stored in the liver) cannot be utilised at all - your liver is too busy coping with alcohol. When the alcohol kicks in to give the BG-lowering effect depends on the person, what they've eaten, their alcohol and insulin tolerance levels, body size, activity etc.
The liver is busy doing the work of processing alcohol, so it can't produce much glucose while it is ridding the body of alcohol. While it is doing that then Bg levels will fluctuate.
So I would say - go ahead with your glass of Port (small) I do !! Moderation is the keyword here.
Ken.