From a few personal experiences, I can tell you that anytime I took, say 1mg, Xanax after a few drinks I would suddenly wake up in my bed in the morning or on the couch with no recollection of anything that happened 20 minutes after taking the pill. I never remembered any euphoria, conversations, or actions. It was a guaranteed blackout in a tiny blue pill.
Oddly enough, I always woke up hangover free because apparently I would not feel the need to drink any longer. I was known to drink half a liter of booze while in an alcoholic only blackout however.
Nasty stuff. Your mother-in-law and father would need to probably taper off the benzos under medical supervision to be safe. However, there really is no strategy that you can employ. They will need to make the decision to stop taking this drug on their own.