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Where a property is held as Tenants in common, the legal ownership passes to the survivor (B) on the first owner's death (A), and the Land Registry will remove A from the title on production of their death certificate.
However, B then owns the property outright, subject to A's "equitable share". If B has inherited A's equitable share, A is then entitled to sell the property and retain the proceeds of Sale. Likewise, if B has subsequently died, his next of kin/Executor is entitled to sell the property and retain the proceeds of Sale.
Therefore, if your Mother did not leave a Will (or you can't prove she made a Will, leaving her equitable share in the house to someone other than her husband), then under the rules of Intestacy, her husband would no doubt have inherited her equitable half share in the property.
Accordingly, if this is what happened, his new wife would be entitled to sell the property if she was his Executor under his Will or as next of kin if he dies without a Will.
I am sorry this is not the answer you were looking for, but sets out the legal position.