But you can contest an unreasonable Will in England.
I think OP, that if you can afford to, you should see a specialist solicitor asap about how likely it would be if you were to contest your mum's Will after she dies, that her treatment of you could lead to her being treated as if she died Intestate?
I know that it would probably cause you a lot more hassle now, and/or when your mum dies @Narcfamilies, but if I were in a similar situation to you, I would be so upset by the way I had been treated over the years by such nasty family members - who are supposed to be part of your strongest network of love and support - that contesting the Will, whether I won or lost the case, would help me to feel so much happier within myself.
I am not usually a vindictive person, but your mother (particularly), has taken most of, if not all of, the pleasure, security and comfort from your childhood, so you may well need to feel as if she and your sister did not get off completely scot-free in the end.
However, I feel that I need to add a couple of addenda to my paragraph above:
If you know, or seriously suspect OP, that keeping or having any further contact with your mother and/or sister, will cause you even more damage, and will make your pain even worse, then do just go NC with them straight away, and ignore my suggestion in the paragraphs above.
You are the one who has suffered for all of these years, you must put yourself first for once - occassional and necessary (apparent) "selfishness", is not a horrible trait, it can be a life saving one! You coming through this traumatic and extended period of your life, with no negative feelings towards them, and with your mental health intact, and with you also managing to be a loving and caring person - which you have already cracked - would, of course, be the best result for you. You may need some counselling to help you achieve the best outcome and in the quickest time, but that is fine, after all, it is why we have counsellors in the first place 🌻🍁xx
Ps. Whatever you decide to do at the moment OP, please do not delete the message from your mother that was meant for your sister but sent to you by mistake! In fact, please try to find somewhere safe to keep it.