wether she will be forgiving or not is entirely upto Ekta. But I personally feel that she shouldn't the reason is:
ONS can be forgiven, don't get me wrong, forgivig something like that isn't the easiest thing to do, but ONE mistake can be forgiven. Even if it hurts, if u really love the person
Both Jai and Pia made the mistake, while jai regretted and admitted his mistake, Pia didn't. she turned her sister's life to a hell. she did what ever she could to to keep jai and bani apart, even though she knew that they loved eachother. She even got bani blamed for kala jadoo. She yelled at jai for saving bani from the fire.
So what happened after ONS is what makes pia's deeds not only mistakes, but crime ( not in legal manner) I mean "galtiya maaf ho sakti hai, jurm nahin"
So if bani really love jai, she can and is trying to forgive jai. While its harder for her to forgive pia, cause of what happened after ONS, especially since she is her blood related sister yaar.