I trust Sona's judgement for values...
She knows exactly how and what blessings and values mean...
She values Ishwari's blessings as well her parents' dreams.
Her father had dreamt Sona in that saree, her mother in law had taken blessings from God and gave her blessings through it.
And Sonakshi values BOTH of them.
Since this is a fusion wedding, what's the harm in a fusion dress?
She wore the saree her father brought for her, and wore the Chunni of the blessed dress on her head (since putting something to your head means giving it utmost respect)
She can't disappoint the both of them, and this was the best solution she could find.
When she will explain this tomorrow, I am sure she is going to earn brownie points from both the ends...
PS: My take.. Agar aisa na ho toh maana mat...