How do I help my sister to stop feeling like a victim?

Oh dear, old hands again

Any suggestions we make on how to resolve things are met with anger and defensiveness.

She will not go to counselling nor will she use this or the other online support group I found her in the hopes of getting some support from those in her situation who may have lots of good advice.

She has now started to demand that we do not speak to her daughter until her daughter speaks to her even though I have explained that her daughter says nothing I'll of her and we do not discuss her.

I just don't know what to do

Your sister must be heartbroken, I can’t even imagine how it must feel to be estranged from an adult child and with each passing month and year it must feel worse rather than better, as the chance of a reconciliation moves further away.
But I understand how very difficult this must be for yourself and the rest of the family who just want to help your sister cope with this situation and move on with her life albeit not the life she imagined.
I am not estranged but went through a bad situation in my younger years when my loved ones tried to persuade me to reach out for help, for a long time I didn’t until I was ready to do this for myself. Your sister also will probably reach out when she is ready, am afraid you can’t force these things.
However, you must kindly insist to your sister that your relationship as the aunt of her DD is not up for negotiation, and that you will continue to speak to her as usual.
Hoping you can navigate your way through this storm and am sure others with experience of estrangements will be along soon 💐

Just do what you’re doing. Still speaking with your niece, still conversing with your sister and other family members.

This is between your niece and your sister and no-one else

Your sister's daughter has escaped her control. Her life is out of control.

To compensate, she's trying to control you (and whoever else "we" encompasses). It won't work.

The only thing your Sister can change is herself. Until she realises that, nothing will mend.

The message you /"we" need to send is that you can't change her life, or her daughter. It's hard, but it's true.

"Stop looking back. Don't keep telling US what to do.

Focus on you, and your future. What are YOU going to do? "

Stop making suggestions InRainbows; if and when your sister is ready for help and support she'll seek it out.

Firmly and with kindness, tell her you will not stop talking to your niece and I say with kindness, because it can be very hard knowing that a close friend or relative is in contact with the AC whose estranged you and if they're seeing the GC you can't see, it's even harder.