Pari found out Rajeev cheated her and married her for property but she also knew that Neeti was unaware about everything so uska koi kasoor nahi tha iss betrayal mein. However, Pari has knowingly betrayed Neeti by hiding the truth from her. If Neeti had cheated Pari, then Pari would probably have still helped Neeti because that's just how dumb and naive she is. She lets everyone use her and walk all over her.
Even if Pari wanted to save Sanju-Neeti's marriage and hide the truth from Neeti, then why didn't she take divorce from Rajeev when she learnt he was married to Neeti?
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