1. The Relinquishment Deed is compulsorily Registrable. If not Registered, then you will not get title for the remaining 50% share.
2. First ascertain as to what is the Circle Rate/Guidance Value for registration of the 50% share in the property from the Sub-Registrar's office. They would be the best source of information/confirmation for this.
3. If the value of Rs. 14 lakhs which you are willing to pay and which your co-sister is ready to accept, is less than the Circle Rate/Guidance Value which would be adopted for the purposes of Stamp Duty, then the difference between the Stamp Duty value and the actual consideration which you pay would be treated as (i) income from other sources in your hands and the same would be taxed in your hands; (ii) the stamp value consideration would be treated as full value of consideration and the capital gains would be calculated and taxed in the hands of your co-sister.