She has a bit of a substance abuse problem and can be quite unpredictable. She has to drink in order to be in the room with me when we have an altercation.
Please let me know if this is a rational idea or if I am acting out of emotion, I am trying not to let my anger sway my judgement and I understand how she is unable to regulate her emotions. I don't think I can keep using it as an excuse to let her get away with how she treats me and now my soon to be family.
I want to key in on some of your statements. "A bit of" is minimizing the substance abuse. It's not a "bit". It may not be her main underlying issue but it is a significant one- no matter what the amount of substance is.
She "has to" drink in your presence. No, she doesn't "have" to because of the circumstances. She is the one responsible for her drinking, even if she herself isn't able to control it.
You "pray your parents will understand"- if they are enablers, they may not understand, but this isn't a reason for you to have boundaries on your own well being.
You are not being irrational by choosing to have boundaries. Your plan to protect your future and emotional well being is rational and reasonable.
I'm not being critical of your thinking and FOG. I recognize this as similar to the thinking I grew up with, and this is why I point them out. When we grow up in a family with dysfunction, it's what we know as "normal". The uncertainty, FOG, self questioning is a result of that. However, you also have the insight to know this isn't functional and are working on positive change. This is a good thing.
In a family with dysfunction, all family members take on behaviors that keep the family in balance. If one member changes their behavior- the other members feel a sense of discomfort. They may then attempt to get the family member who changed back in their role. If the family member does not do that, they may get angry and even reject that family member.
It looks to me as if your family pattern has been to enable your sister. This is common too- as it helps keep the stress down. Everyone benefits when you appease your sister, even if it isn't to your benefit to do so. Why would parents do that? Possibly because these patterns can be intergenerational. It may be the one they know as "normal" too.
You can be the one to break the cycle for yourself and I encourage you to do so.
I also stepped out of the family pattern. I began to have boundaries with BPD mother. I also assumed my father- who was her enabler would understand. I wish he would have, but he didn't. Still, it was important to me to make this change.
I also had enabling and co-dependent tendencies, people pleasing and had to learn about boundaries. I did counseling too. It wasn't until later that a counselor recommended 12 step groups in addition to the counseling. I was surprised that nobody had mentioned them before. BPD mother did drink, but it was a long time ago and substance abuse was not an issue in my own family. What I realized though is that the family patterns are learned in childhood, and also are similar in families where there is a person with a disorder. I found groups like CODA and ACA (for adult children) to be very helpful in addition to counseling. (They don't take the place of counseling). You may want to check them out.