Prachi did not know what love was until she fell in love with Milind. She mistook her close friendship with Neev as love. So once Neev and Prachi got married off to other people Prachi had to forget her 'love' for Neev as she was now someelse's wife and he was married to her sister. As Milind's wife she learnt that love is more than friendship its a passion and intensity shared with someone who completes you. She has truly loved once that is Milind and realised what she had with Neev was nothing more than love between two friends. I think it is evident when comparing scenes between Neev-Parchi and Milind-Prachi. Also another point is that why is no-one questioning how Milind forgot his first love, Ayesha, whom is went to so much truble for?
(This is my opinion and do not want to offend anyone and if I do I am sincerely sorry. )