I want to provide a bit more practical legal advice as I’m not sure what else I can add and I really don’t get the sense you are ready to go NC. However like others I really hope you do. It sounds like getting some counselling would be really helpful and maybe give you some clarity on how to move forward. I’d recommend you contact a local domestic abuse charity for this if you cannot afford private, the waiting list would be long but you’d hopefully get longer term support than the NHS, plus someone that understands that this is domestic abuse. The same goes with finding a private therapist, find one that has that experience (happy to help).
Domestic violence and abuse is defined as:
"Any incident or pattern of incidents of controlling, coercive or threatening behaviour, violence or abuse between those aged 16 or over who are or have been intimate partners or family members, regardless of gender or sexuality. This can encompass, but is not limited to, the following types of abuse: psychological, physical, sexual, financial and emotional."
To be this is clear psychological and emotional abuse. If your mum didn’t also sound very unpleasant I’d actually see your sister as the primary abuser, as in some ways your mother seems very frightened of her and the consequences of disobeying her (e.g. being terrified of you turning up at her house when your sister might see). It seems like your sister has had an extreme amount of power and control over the family for a long time. At this point you have nothing to lose and I would be tempted to ask your mum if she is afraid of your sister.
You also describe that when you try and reduce contact and block them on certain social media that they continue to try and contact you by other means. This would meet the definition of stalking or harassment (I would need more information to distinguish). If you decide to cut all contact and make that clear to them, any further communication by them can be considered harassment at the very least. This includes phone calls, text, emails, visits to you house, letters, malicious Facebook/social media posts where you are named or identifiable. The police are getting better at handling this and if a crime is committed they will step in, but there needs to be a clear boundary that you have tried to put in and no communication on your side for the most success. There are specialist domestic abuse offers who you can ask to speak to. You could also consider a civil injunction against them which if they break they would also be in break of the law.
This might all sound extreme and these are all options you may not want to take, but I want you to be prepared and also feel bolstered by the fact that there is support out there and people able and willing to support you. I have a feeling this may escalate if you try to cut contact, however much they claim to dislike you they clearly like to have power and control over you, and losing that is often a serious catalyst, as it seems to have been in the past. You will likely need support, as would anyone in this situation.
I’m sorry if you know all of this, as you said you work in mental health nursing, but often we can’t see our own situations clearly and family have a way of making things very confusing, especially when we are told narratives our whole lives. You might want to read some R.D.Laing - his books on madness in the family and how one person is forced into a position of ‘madness’ seem to be appropriate here. There is a little bit writing on sibling abuse online as well, it is worth reading.
I have not been in a situation as extreme as yours but my relationship with my family has been difficult, especially with my sister. It still lead to me entering abusive relationships. It took me a long time (and a huge breakdown) to distance myself from them and get the space to work out my issues. They will never understand my experience of growing up and most people with happy families never will. They try to say well meaning things but they have no idea what it’s like to grow up in a dysfunctional family and the damage that can, has, and will continue to do. So keep this thread in mind and make sure you talk to the right people, your friends sound supportive (my family look ‘perfect’ on the outside and they would do their best perfect routine so none of my friends understand).
If you really do still want a relationship with them then I think you would need to consider family systems therapy as your bottom line going forward, I’m not sure if the NHS would offer if the children aren’t involved, so then you’d need to all be prepared to pitch in and pay, not just you paying for it. If they really want you in their life then they’ll help pay for it and make it happen. If not then that’s their loss.
Thinking of you x