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Hi, thanks for your enquiry. I am afraid that if Contracts have been exchanged, there is a legally binding agreement between your Mother and her Buyer and as such, your Mother is unable to withdraw from the transaction now. Whichever Solicitor is acting for your Mother would have made sure that your Mother was aware of this (although I do not know if any family member has Power of Attorney, but if they do, they would have had the right to sign the Contract on behalf of your Mother).
Although I appreciate you are in line to inherit your Mother's estate, this makes no difference I am afraid as a Will only comes into effect when your Mother dies and only applies to assets owned by her at the time of her death.
I am sorry this is not the answer you were looking for, but if I have assisted, I would be grateful if you could rate my answer.
Kind Regards Al