I understand how hurt, a get and upset you must feel but I really think these feeling are totally clouding your judgement.
You admit yourself that your relationship with your mum wasn't great and you wrote that all you wanted was for your mum to love you, so clearly there were issues.
Maybe she didn't approve of your life, the choices you've made. Maybe she thought that you were trying on her passing away to better your life. Maybe she thought that you were not a good carer when you were looking after all.
How long ago was the mirror will drafted? Maybe your brother influenced your mum. Maybe he convinced her that she wasn't well looked after and that was the reason why she agreed to move with him.
Ultimately though, influencing doesn't have to mean manipulated or coerced. You would have to evidence actual emotional abuse leading to her feeling in fear if she didn't agree to change the will. How do you intend to prove this?
Your view on the fact it was your brother's solicitor who did the will show how cloudy your judgement is. He is a professional and wouldn't be manipulated in doing something illegal.just to please a client when there is no benefit to them. It makes no sense.
Don't forget that if you challenge the will your brother will have a say too and he could brings things up that will come with yet more upset and pain.
Ultimately it is rely sad how children become so reliant emotionally on their parent will, ie. Money that was never theirs in the first place. You really to move on. You can make your life a happy one without this money and what you see as a betrayal.