Since the object of the settlement deed was to come to to effect post death this deed will be treated like a testamentary document, in other words like that of a will. So there is no bar in your mother dealing with the property. However since this is happening out of consent, it is better to avoid complications. The best way to avoid such complication would be if you furnish a Power of Attorney to her giving her the right to alienate the property from your end. This should ensure there can be no complications at all. But then again the POA is not necessary and she can sell it now as well if she has the entire share of the property.