I think rather than seeing as Meghna snatching Naina away from Karan, it's actually an elder sibling who is already feeling the guilt of being responsible for her sisters forced marriage.
The problem is that like Meghna has always said, Naina doesn't express her feelings much. How I see it is that she just goes along with the flow anyway. It kind of makes me feel sad that she readily accepted Karan as her husband without knowing him or anything. She just wants to do her duty so illness or no illness, friendship or no friendship...Naina was fine staying in this marriage which makes me a bit uncomfortable (as adorable as Nairan are). For Meghna, she's just been constantly lied to and she's fed up of it. To her she is just seeing her younger sister continuously compromise when she shouldn't have to.
Also, this show is more about Meghna-Naina than Meghnal or Nairan right? So high drama has to come from these two and how if affects the other sister.