I'll start with a value of mine, the biggest value I hold. It helps me define boundaries. I have envisioned this as a flow chart - pyramid style, flowing from the top down. WIsh I could draw a diagram... ; )
Treat me with respect and you will receive the same in return. Disrespect is not tolerated.
Now that said, I've had to define what respect means to me. It means autonomy (I am in charge of my choices in my life, not anyone else), the right to privacy (hands off my stuff, folks. If I want to share, I will share. That goes for property, money, personal items, thoughts, opinions, etc.), the right to peace and serenity (even if it means going no contact with someone upsetting my need for peace and serenity). There's more here....just not finished with that first cup of coffee yet.
Now underneath those would be further defined behaviors I won't tolerate. Such as anger and blamestroming, not taking personal responsibility, abuse of any kind, being under the influence, etc...
Ok, so we define our boundaries...then what do we do? Standing firm doesn't mean forcing them down someone else's throat. That's the tricky part for me. Standing firm on my boundaries without coming across as aggressive and mean myself. What I've learned in Al-Anon is to stand firm with love and compassion and empathy for the other person I am struggling with. I can protect my boundaries while also seeing where the other person is coming from. Just yesterday, my RAH called me in the morning to see if I wanted to go for a hike. I did, but it was raining so I told him maybe we should wait...maybe tomorrow...well he got angry and abruptly hung up on me. I didn't call him back. I didn't get mad in return. I just thought "wow he's grumpy this morning. What a butthead" and went on with my day. He called an hour later to apologize, took personal responsibility for his emotions and behavior, we went for a hike and had a nice time. Now, if he had called back to berate me for causing his bad mood, I would have hung up and gone on with my day. Because that's a boundary - I am not responsible for your feelings. You are. Own them. Or leave me alone.
You are heading in the right direction by understanding where your Mom is at right now. Mother's guilt can be a powerful thing. She sounds just as addicted to your sister as your sister is addicted to her substances. That addiction really clouds rational thought!
The last thing I will say about boundaries is that you have to stand strong...some people don't like boundaries and won't respect them, especially if they don't even know what boundaries are to begin with. Be prepared for flack. This is where I remind myself "what other people think of me is none of my business".
Addiction creates chaos for the entire family. I am sorry you are having to go through this...you are not alone...and it is not easy.
~T