I understand Sunaina's point here, she being a mother is hell scared of the consequences which this revelation will bring in.But disclosing the truth to both Veer and Krishna would have made the situation much easier for KrishVeer. Now as Krishna is in jail , the situation is much more serious, previously it would have considered as accident, now it will be considered as murder.
Also Krishna knowing the truth from Veer/Sunaina or knowing it from outside will make a huge huge difference. If she would have heard it from Veer or Sunaina, she had a chance to understand their situation better and we can expect her to be more empathetic in that case. But if she hears it from outside , I am not sure whether she will be that empathetic, she may feel like Veer/Sunaina has covered up the truth and she may feel cheated. krishna's approach is very important here.
For Veer also situation is same. He would prefer to say the truth to Krishna by himself rather she knowing it from others.