While making this plan, Abhay and his sister had one plan. It was to hurt and humiliate Diya. Make sure she has no face in the society.
I see a few problems with this plan.
They have not really understood what Diya has become in all these years.
In real life, even in villages, they will loose the social sympathy which they received when Diya left.
Diya at least had a good and noble reason to leave, while they are being petty, vindictive and selfish.
It is true that women with broken marriages are treated badly by society but the kind of social circle they are from, what are the odds someone else will be ready to make an alliance with them.
No girl's father will trust them. It is not just girl but her whole family is dishonoured by them. And what they did will be considered not Kshtra dharma. They back stabbed.
And the worse blow to their plans is the Ace up Diya's sleeve which no one (even herself) knows of. Ratan!
I do not think Abhay and his sister accounted for Ratan stepping up to marry Diya as they dismissed him as a 21 year old kid who is just being enthusiastic about this wedding.