Vividha has already made it painstakingly clear that she was miserable during her brief marriage to Ravish. How much more misery will she undergo therefore if she has to spend an entire lifetime with him? Is this what a loving Atharv would want for her, a lifetime in purgatory? Also if he genuinely regards Ravish as his brother, how can he want a lifetime in purgatory for him too? It's really painful to get what you want but to never have it. Look at Suman if you need an example. Here, even if Ravish gets Vividha would he ever really have her? How painful for him and what a lousy thing for one brother to do to another.
True love never waivers no matter what it faces. If Atharv believes Vividha's love is true he ought to know that no matter what he does to make her hate him he will not succeed. She couldn't hate him when all indicators were that he had killed her sister, her own flesh and blood, so it's clear there's is really nothing he can do to make her hate him. So why go through this futile attempt if it's a non event.
Another thing in his so called great sacrifice and attempts to force a Ravidha relationship, Atharv is taking away her right to to make her own decisions. I won't go into the whole rant she made about this but yeah he ought to know better. There are billions of men out there. Ravish might not be Vividha's cup of tea but who is to say that out of all those billions there isn't another who might eventually stir up if not the same, then a close modicum of the kind of emotions Atharv stirs up in her.
As he now knows and as the saying goes, life is short. In my view, if Atharv truly loved Vividha he would marry her and in those few months he has left of life he would give her such immense happiness it would last her an entire lifetime. Already Vividha has demonstrated a willingness to live as his widow without the marriage, how much more of a merry widow would she make having experienced marital bliss with Atharv. In fact a widow who has experienced marriage happiness is more likely to be open to another relationship on the chance of getting another one. Who knows Vividha might turn out to be one of those and Atharv could still get Ravidha after his demise.