There is clubbing provision where though the asset is transferred to other, the income from such transferred asset is to be clubbed with the transferor.
In your case, it would be necessary to see who the actual owner of asset is. Is any income from the flat (like rent) shown in your sisters hands. Wondeirng what was the purpose to show her the part owner of property. Has she paid you anything for being part owner ? IN case you are the actual owner and paid for acquisition of that flat, the gains from sale of this flat will be taxable totally in your hand.
Merely gifting 50% now by your sister does not change any position. If she is the real owner then she also need to show part capital gains. Else show the entire gains in your hands.
Also, it would be necessary for you to maintain all the documentation to justify the position you take for tax purpose. Please maintain documents for ownership with bank entries and sale documents. IN case your ned detailed discussion, we can have a call.