Thank you for your question. My name is XXXXX XXXXX my goal is to provide you with the best possible answer.
To achieve that goal it may be that I will find that I need to gather further information from you. As well, it's quite possible that you may feel the need to ask me additional questions for further clarification.
But, please understand that as a careful lawyer I will always give you an honest answer even if the answer is sometimes not what you were hoping for.
It would definitely be worth your while to take the Will and the letter and see an estate lawyer face to face. This seems like it could be a very blatant case of undue influence because of the fact that you've essentially been disinherited.
So there is no reason not to at least explore this further.
Proving undue influence is hard but if you and your sister both had relationships with your mother it is hard to see how this could have happened. Had you been completely out of your mother's life that would be a different matter.
You can contact the Law Society and use their Lawyer Referral Service. You will be given the name of a lawyer and can consult with the lawyer and the first half hour will be $30.
Here is a link to the contact information you will need:
Let me know if you need any further clarification.