If the first owner is alive and he is ready to make the document and sign the sale deed and appear before the registering authority then she can sell the land.
The reality is that the sale deed need to be registerd and properly stamped but your father had executed a dcument that is not valid sale deed. However like her sister sold the land she can also sell the land just the first owner need to apeear and sign and get slae deed registered.
Father is no need to appear as he is not owner as per requierment of valid sale dee.
However if you wish to do it other ways that is lenghty by filing a suit for declaration....