I feel that Kabir is Saras brother because as soon as Saras says that before my marriage my fear of loosing my dear ones as I lost my mother there was a sudden break by Kabir pushing Saras and Kabir with a sad face on face or you can say anger on Kabir face. Saras asked Kabibir why such a sudandden break he said that we have reached school.
I feel that Saras mother might be alive becase the anger on Kabir face showed as if Saras mother is alive only Saras doesn't knows it.