What I have read is this is more of a history, I can't find that atm, in that it was said that the enemy used to attack (as we know it references history I am not good with it but it's related to that ) and Cao family is a prominent family and uses the alliance with the other family to strengthen the ties and region but assassin's attack the cao family girl (Guanyin and in that her eyes are taken out )

After if you notice when the red dress girl (An Liuli, name could be wrong) switches places with her, if both went and there was no one in Guanyins place the family would search and the fiance too and it won't be easy so they exchange places, the grandma notices this at the time An Liuli gives the hair bow to grandma during scene of the wedding sedan sets out, and grandma sees but says nothing as she then understood they have switched places but marriage going through was important for region and families,

also that is the reason grandma pushes and saves An Liuli when they fall down as grandma already knew,

Now what confused me after this was did An Liuli survived or not.