I am so sad for you and your family, but thankful that your mum was unable to realise just what a nasty cow your AW was acting like. It says a lot that an old lady with Alzheimers still has more social skills and manners than her D-I-L who is supposed to be OK.
Truly the words and actions fueled by alcohol are legion and mostly offensive or damaging to those who love them.
Sending you a BIG hug and wishes for peace in heart and mind right now.