If her medical records do reveal that she was an alcoholic - would that make you feel better or worse about her death?
Will it trigger other thoughts or questions about your mum and her death that may upset you even more?
I do think you are entitled to know.
However I think you have to ask what will the answer to the question leave me with?
Will it make me wish you had done more for her?
Will it cause you anger that she hid her drinking from you?
Will it change how you remember her?
Grief is a process and there are different stages.
It might be you need to wait and think about getting the answer you need.
There is a part of me that does think your Aunt is right - remember the nice memories you have of your mum.
I really wish you and your family well.