after the death of your father as his legal heirs, your mother, brother and you are equal share holders to the said property. However you and your brother have consented to the transfer of mutation to be registered in the name of your mother. accordingly she is now the single owner of the said flat. However at the time of sale, the buyer in order to protect his purchase and interest is insisting for an NOC from both you and your brother.
Here in Bangalore, at the time of registration of the final sale deed, any party interested in the property to be sold, will be made a Consenting Witness to the said sale. Accordingly you and your brother can be the consenting witnesses to the sale at the time of final registration, your signature along with photo and thumb impression will be taken on the final sale deed. This will be more than adequate and an NOC as sought for by the buyer will not be required.