Jiji wants to take revenge on Nahar & Soloni for 2 reasons -
Reason #1: Soloni & Nahar were the people to expose her intentions in Singh Parivar and spoil her plot. So she wants to make their life miserable
Reason #2: She is still hoping that Tara will accept the baby and give all rights as Brijesh's son. If Soloni is Preg and gives her baby to Tara Jiji's son would have no value. So she wants Soloni to abort her child.
She just wants this family to undergo a misery life and break up the unity and that would be an inderect revenge on Tara.