From what I have perceived of Avantika's character, she is a very black and white kind of person, with no shades of grey in her character. She has always been very direct and straight forward in her actions, so when she loves someone, she loves them with the core of her heart and when she hates someone she does that with full conviction as well, so one would think that since she hated Pankhuri, how could she accept her so easily ? In my opinion, she never hated Pankhuri, she just disapproved of her and PaYa's relationship, and that too only because she saw herself and Harish in PaYa and thought that since she and Harish could not make it work because of their ego and differences, neither could PaYa, but like she said tonight PaYa and Pankhuri proved her wrong, because it is not necessary that two people who are totally different from each other cannot make go of a marriage and live a happy married life, but it really depends on what sort of upbringing and values you have been brought up with which helps u tremendously in one's married life and Pankhuri was very lucky to have been brought up in such a loving family, with such loving and sensible parents who taught her the meaning of relationships, family and husband, but Avantik and Preeti were not so lucky, they lost their mother at a very young age and grew up without their mom and with a dad who were too busy with business to look after them, but Avantika being the first born, she was always very emotionally dependent and seeked attention with her father gave her, so she grew up with no one saying NO to any of her whims ever, whereas Preeti who was even more starved for love, always silently pleaded for her dad's and elder sister love, but never got it so like most younger siblings she became emotionally independent and made a separate life of her own, so it was no surprise that she seek love outside and made a wrong decision.
Avantika realized today that the amount of sacrifice PaYa made, especially Pankhuri to make sure that the misunderstanding between herself and Harish is eradicated for good, because Adi being the son of the house, he would never been thrown out of the house or from Avatika's life but Pankhuri who Avantika disapproved of at every turn, she still went ahead with the plan of bringing Kailash back to sort out all the misunderstandings and reuniting their family. That is why Avantika tonight had no qualms in accepting that she was wrong about Pankhuri, because that is how Avatika is, if she believes someone is wrong, she would go against them every step of the way, but if she believes or is made to believe that someone or something is right, she would stand up for that reason or for that person and fight against the world. Like I said, her character is either black or white, she doesn't have grey shades like Sheila and Co.