Sounds like a lot of fear is being triggered by this visit. And fear is one of the most wearing emotions for us.
As far as not being able to say where you're going, and that making it hard to get to meetings, I reckon you can work round that. Or even just be honest to a point. Did they know that you used to drink a lot or heavily? If so, the you can tell them that you decided to stop drinking and you go to see some people who help you with that. You don't need to say the scary words like Rehab or Alcoholic or AA. To be fair, if there has been a diagnosis of alzeimers then they might not be as interested in where you pop off to as you expect. If you don't want to hurt them or worry them, what about a complete fib (with the best of motives of course) such as saying you're house / pet sitting for a friend who's on holiday. Obviously make sure that your husband is aware of where you're really going.
Alzeimers can be extremely stressful to deal with, but do try to remember that alcoholism is equally stressful. The last thing you or your husband needs right now is a relapse . And, there is no such thing as a good excuse for a relapse. We can deal with things better sober than we can drunk. So in times of stress its important to stay sober. I can only imagine how sad and scared your in-laws and husband must be feeling.