you have yourself mentioned that its Amrit's ego and jealousy which makes her hate her sister and want to take revenge, may be, for being the chosen one...
you are right, in the first place, she only wanted to marry Agam for the money they wud get to save them from total bankruptcy... slowly she started liking him ... and now she just wants him because he is beyond her reach... Amrit is currently being ruled only by her blind jealosy and the sky high ego...
Agam might have humiliated her by scorning her and marrying her sister... that in itself is a big blow to her pride..but what angers her more is her feeling that her sister betrayed her by agreeing to marry Agam. She does not know about the promise Nimrit had made to Agam in the temple and the emotional blackmail made by rajmata threatening to commit suicide... She knows of her sister's love and devotion for her but that does not make her humble enough to forgive her and look at the reasons her sister took the extreme step... on the contrary she makes that very love a weapon to destroy her... that's the extent of Amrit's bitterness...
what happened to Amrit is the result of agam's revenge and that should have taught Amrit that revenge begets nothing but more negativity and ends up hurting everyone... but she is governed by her anger and jealousy can see only her need to satiate her burning need for revenge...
Whether she takes revenge from Agam or Nimrit... they may or maynot get hurt, as their love will ferry them across this river of revenge and hatred... but amrit surely has set herself on the path of self destruction... which she does not realise or maybe just does not care...