I have found that when I stick to “I” statements instead of pointing blame with “you” statements it’s much easier.
Examples of things you can say:
I have thought about the plans for you to come stay with me and realized that I am happy living alone and wish to keep it that. I have found that living on my own allows me more freedom and allow me to shoulder all my own responsibility keeping me independent instead of dependent on anyone. I have found that any disruptions from my normal routine are stressful for me.
This way you are stating how you “feel” and if she pushes you then she is not thinking/caring about you only herself.
Keep reminding yourself that her choices were her own. She chose to remain in an unhappy marriage. The fact they have no equity in their own home is NOT your fault or responsibility.
Trying to figure out solutions for her problems is not on your shoulders. Her moving in with her son is not a healthy solution but that is between them and has nothing to do with you. It’s not even your place to suggest it.
I might suggest instead of focusing on her drinking or not drinking issues that you focus on your codependency and inability to stand up for your own needs and wants.