1. More facts are required to be known for advising properly,
2. You stated that the property was in the name of her husband and she got it in her name after the demise of her husband,
3. How did the lady get the property in her name? Was there a Will executed by her husband? Was there a gift deed executed or registered in her name? Or she was just a nominee of the flat in the society where her husband was a member,
4. Is her sons minor or they are major?
5. If she is a nominee then she is not the owner of the flat which is shared by her sons also,
6. If her sons are major then all can execute and register a Gift Deed in your favour after collecting legal heir certificate,
7. If her sons are minor, then she has to apply before the District Judge for permission to sell the share of her two minor sons and only afyter availing the said permission, she can sell her and their share of her husband's property to you.