They have never revealed any specific history between Koki, Parag, Hetal and Chirag to explain Koki's resentment and treatment of everyone.
Parag
Her husband is the artistic type who isn't the bread winner in the family. He only focuses on his interests and doesn't give any importance or support to Koki. During disputes he sits back and doesn't speak up for Koki as she would like him to.
Chirag
he is the older brother so automatically head of the family. Koki doesn't like bowing down to him. Because her husband isn't an earner she needs to take money etc from Chirag. When Chirag does things for the family Koki takes that as an insult or that she is being taunted. Or that Chirag is throwing his power in her face.
Hetal
She automatically has a higher position being married to the older brother. Her style of dealing with things are in direct conflict with Koki. She may love Hetal but she is doesn't like how Hetal deals with things.
Jigar
He is just collateral damage.
Koki wants to be equal with the Mota couple but because of her husband and being the 2nd bahu she has never attained that goal.
In the past we have discussed and speculated that perhaps there is a story behind it all but I think it is just Koki's character. She is a very ambitious woman who is stuck in a situation that she doesn't like. She tries to gain what she doesn't have in her marriage through Aham and Gopi. Kinjal has set her back a lot in her goals!