Why the twin is doing all this? Two theories:
1) Niranjan and Anjali were happy married couple, when the twin was introduced to the couple and he wanted Anjali for himself.
2) The family money is with first male child of Agnihotri family, Shlok's first child or real Niranjan. I think that's one of the reason Varad was forced to think about second child, if I remember male child, after Jyoti's pregnancy was revealed.
This might be really one the last mysteries to be resolved and my guess is Sid and Aastha will do this.
That's the reason probably Anjali's has changed where she cannot help her own daughter, cannot console her son and bring him out of Swati's memory and cannot care for her parents in old age.