I feel they can show is Sirat is Naira,i will explain how? Well Naira already had a blood clot in brain, it was operated successfully but then it could come again right? So she had memory loss in between. If anyone remembers, in picnic too she had a blurred vision for a time being.
Naira and Sirat might look alike, so the one that died would be Sirat and not Naira.. Nani must have told her story about her mother, childhood and all, this she imagined all this and started getting sympathetic towards Sirat and completely got her personality.. The example could be Bhool Bhoolaiya movie, wherein Vidya Balan feels so much connected to story and character of Dancer Monjolika, that she actually becomes one.She talks, behaves exactly like her.
I feel here too it's the same case, Naira might had a memory loss but this time Nani told her she is Sirat and she imagined everything about her so much that she even adapted her passion of Boxing ..
I feel they can show it this way, coz young Sirat doesn't look like Ashnoor or is not Ashnoor, I agree Ashnoor has grown up but still... So this is imagination of Naira only who is currently Sirat