All i can think for them is that maybe Keerti had to take care of Amma alone since they had neglected Raghav so she got tired of the responsibilities. That is when she met Sunny, who is a free guy and does nuisance. She felt free around him like out of a cage. So apparently Sunny became a drug for her to escape from the world and hence she is not in love with him but she ia extremely addicted to him.
He made her do things maybe that she could have never been able to do.
This is just my theory for their relationship because i have seen a lot of couples like this.