Macushla,
Bless your heart. How horrible for someone to say the things she said to you.
She certainly does not have a realistic view on the situation. Im so sorry she spewed her emotion onto you. She is not quite seeing the situation for what it is, it would appear. Mothers of alcoholics sometimes have these views. I was lucky, as that was not the case in my experience, although I know it is more common to experience what you have than what I did.
Perhaps she will come round and realize that she is wrong. Although in the meantime, is it necessary for you to talk to her? Have you considered telling her that her presence is important in your sons life, although you will not tolerate her berating you?
I am sure others will be along to share what worked for them with the in-laws and their children.
In the meantime, blessings and hugs to you!