In this situation there is no need to take signatures of the sons of deceased sisters. This was an ancestral property before the partition. After the partition among the legal heirs it became the self acquired property. and the owner has separate possession.
This property has already been divided among the legal hairs therefore the nature of the property has changed. Now it is a self acquired property..
Signature of deceased sister’s son is not necessary
The provisions of Section 6 will not apply on this property because The property was divided before coming into force of the Amendment Act 2005.Before the Amendment Act daughter had no right in the ancestral property because they were not treated as coparcener in the joint Hindu family.
The facts of your case suggest that you can purchase this property without taking permission of other persons. The owner of this property has the exclusive right to sell this property. Owner of this property has a sound mind therefore he can execute the sale deed. The son of the deceased sister has no right or interest in this property therefore his signature is not necessary for effecting the validity of sale deed.
If this property would be in the joint possession of coparceners then the son of the deceased sister can claim partition of this property.