1. Neil did not choose to marry Avni. She herself alone made that decision (or rather circumstances). And that's why in her mind she herself is taking the decision to stay away from him. Remember that he hasn't confessed his love so she doesn't know that living without her is a big punishment for him. She is not smart enuf to fathom his feelings and he hasn't told her also. 2. On the other hand Ashish not only had Aysha as an unmarried wife but had two kids with her. At that point he is much more responsible for his actions and him choosing his mother is catastrophic, especially for his pregnant wife and for his 12 year old kid.
In other words, Avni doesn't want to go ahead and reach that stage where Aysha and Ashish eventually reached! She doesn't even want to take that road.
As for her feelings towards her father- She probably understands her fathers dilemma now but he didn't stand up at all for his wife and children when they were totally dependent on him and that destroyed the whole family. So how can it be easily forgiven by the one who survived the whole tsunami ??
I don't know if I conveyed it ... but I can totally understand Avni's POV.