Hi, I am new to this forum too, as the wife of an alcoholic seeking support.Sadly, after reading your post, I do think your daughter in law is an alcoholic.The giveaway is the classic having a drink first thing in the morning, and she obviously knows she has a problem, why else hide it in a cup?
I wish I could tell you some magic way to cure her, but I can’t.Have you spoken to your son about your worries? He may need your support.
Best wishes and good luck!