immediately after the death of the father, all his 3 legal heirs, namely, his wife, son and daughter will be succeeding to 1/3 share in the property. hence, all the 3 legal heirs are to sign and convey their 1/3 rights to the purchaser, if not, the ownership will not be transferred. By mother executing the sale deed, she has transferred only her 1/3 rights; and the remaining 2/3 rights stands still in the son and daughter. it is to be noted that, it is the self acquired property of the father. hence, in my opinion, merely by getting the signature of the son as a witness, his 1/3 rights will not transferred to the purchaser. So, to cure the defect in title, both the son as well as the daughter shall release and transfer their 1/3 rights each (together 2/3 rights), by means of a registered release deed, and in no other way, the 2/3 rights held by the son and the daughter could be transferred and vested in the purchaser.
On the other way, the son and the daughter of the earlier vendor, can be arrayed as parties /vendors in the present sale deed which is to be executed in favour of the present purchaser, and their 2/3 rights could be transferred to the present purchaser. why i suggest this is to save the stamp duty. becoz, in karnataka, when a release deed is to be registered in favour of any person other than a family member, full stamp duty on market value (2/3 of guide line value) shall be paid. to save that, the said son and daughter can be added as vendors in your sale deed.