Do you have the payment detail with you i.e., the payment made by your mother to the vendor or your aunt?
If you have that you may issue a legal notice to the aunt as well as to the vendor who executed the registered sale deed on the name of aunty alone demanding him to cancel the said sale deed and re-register it on both the names or to refund the amount taken from your mother with interest. If there is no response you can lodge a c police complaint besides filing a suit for cancellation of the said registered sale deed ad an injunction application to restrain the defendants from further alienating and encumbering the property till the disposal of the main suit in the civil court. Consult a local lawyer and proceed.