I know everyone is upset, angry and very disappointed with Akshu but what if this is the reason:
Abhi's feelings weren't one sided she has feelings for him too, so she is doing this for aarohi. Akshu's character isn't this cruel, even if she is doing it for her sis she wouldn't go to this extent; to reject him in front of everyone, she would want to do it privately.
I mean she can sacrifice that's expected from her, but reject in front of everyone is so not like her. That's why I can't help but think that she was forced to humiliate abhi in front of everyone by aarohi so that everyone in Birla family/Abhi hate her.
So I feel that aarohi threatened and blackmailed her with sirat's last wish which is cruel and ruthless of her to use her mother's last wish to her advantage and separate her sister from her love. And we know that akshu feels guilty even through it's not her fault. And that's the only excusable reason I can think of, which doesn't ruin her character and is consistent.
And if Akshu is doing it for this reason (quote above) then I won't blame her. She is not doing it intentionally or for her own reason (to make aru happy). She is doing it because of her mother's last wish, and I will feel proud of her.
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