i've read sumwhere that kabir is zareen begum's step son n' she killed him cuz she was jelous of fact that he got the most attention in the family, n' the money too, thus neglecting azan (her real son) n' also she wanted azan to marry afreen, so those were her motives for killing kabir.
No, it wasnt Kabir thats the step-son, its Salman. They said it in the show too, that Salman is the step-son