I dont think that Meghna is wrong
Every elder sister who loves her younger sibling the way Meghna does will have the same reaction at first what Meghna has...shock, disbelief, anger
I love Meghna as a character , becoz she's not mahaan like most FL of Indian shows , but she's human and makes mistakes as we do...this is what makes many ppl connect to her
She will feel every emotion, becoz Naina married Karan, only to save MeghNal's marriage, so obviously she will feel guilt with anger, becoz no one cared to inform them beforehand or even after marriage abt Karans condition...not even Naina
I am sure that when she cools down she will think more practically abt it