Hi
First a property in the name of her woman is her absolute property under section 14 of Hindu succession act.
It is immaterial whether your grand mother received the same from her parents or from any source.
There is nothing called ancestral property if the property has been obtained by a woman. the word ancestral property is applicable for property held by a man.
The property in the name of a woman is her absolute property and she can deal with the property in any which manner she wants.
Since your grand mother has transferred (i presume she has registered the property) in the name of your mother in the year 2002, then the property belongs to your mother only.
If your grand mother has transferred the property to your mother in the year 2002, it cannot be reopened now as law of limitation will apply. if at all your aunt were to contest the transfer, she should have contested in the year 2006 latest. they cannot contest in the year 2016.
Your aunt has received consideration and from your query, i understand you have a Rs20 bond paper as a proof.
the bond paper is absolutely valid in eyes of law.
Your aunt's sons do not have any right whatsoever (grand sons, ancestral property etc) and they can go to court if they want. But the courts will dismiss their claim within 3 months and throw their case to dustbin.
Hope this helps.