I understand that she loves her dad and wants his happiness but I'm sure if she tells him the truth then he would be even happier and importantly proud of her. She knows that Naani and step-mum are not all good as they seem which is why she would rather prefer to talk to her dad alone as she knows she can't trust the other half of the family, therefore she's not a fool.
Im guessing she's protecting her dad from the truth, because he will feel betrayed by what his wife has done since he loves her alot and would lose his companionship and family and so on. But Viddhi should think about herself for once and perhaps the truth will help open her dad's eyes and bring him to reality.
If not the whole truth-she should at least confess that she does all the desgins, then make some excuse for Jassi- saying she's also good at her work but didnt reveal the truth to their dad because sJassi did not want to fail him and so on.
What do you think? When and how would the truth come out? More importantly should it be Viddhi who tells her dad all this or circumstances lead to the truth?