Firslty, as per the information provided by you in the present query, makes it clear that property fisrt came to her husband thereafter, his death as no one was there as a male child, so property case to your sister.
Secondly, once it has come to her then it was her property, and she had all right to transfer it to anyone she wished to, which she gifted to you.
Thirdly, you are in a very strong position as to cancel the gift deed in the absence of the person who made it, not so simple to prove it as fraud or by coercion.
Fourthly, you please don’t write anything on anything, just wait for some more time, and you will be in a win win situation.
Good Luck...!