1. The property was in joint names of your parents,
2. After demise of your father, his 50% share of the said property will be devided amongs your mother and all the children of your father,
3. Hope all the children of your father has executed and registered a Gift deed of Settlement deed in favour of your mother transferring all of you shares of the property in her name,
4. If it has been done then your mother is the rightful owner of nthe said property,
5. Mutation means changing the name in land records for paying tax,
6. Mutation does not convey any title,
7. It seems that your buyer is very cautious,
8. Mutate the property in the name of your mother since it is not at all a difficult job.