My mother never remembers my birthday, and my father was the A - she was the codie!! And it is awful, you're right!
Who knows why your SDs mum forgot, alcohol is just a mask and a drug for the problems the A has -so is codependency a mask and a "drug" for the codie. When you take away the alcohol or the codie behavior it's not always some really super awesome person left standing there! Not always the person we would expect.
As adults we can train ourselves not to have expectations - but for children it is just in their nature to expect things. In a quiet moment maybe reassure her that no its not normal for a mum to forget a birthday, and as hard as it is to believe - it is not her (SD) fault, her mum is just dysfunctional!! Hearing that rational true explanation over and over will help her see it is not her fault (it's so easy to feel unloved when slighted like that as a kid.)
Your SD is very lucky to have some stability and loving people to give her a birthday celebration. People to show her daily that she is loveable and important! Don't waste a minute of your time trying to figure out WHY an A (yes, even an A fresh out of rehab!) behaves the way they do. You'll drive yourself cuckoo. Use that energy for something else!!
Happy Bday to SD!
7
Peace-
B