I understand Meghna's fears totally,
1.Even though Karan has been living with Chauhans no one ever was as close to him as Naina is now.
2.Poor Meghna, she's getting nightmares, even I wouldn't be able to bear that torture if I were in her place.
3.I don't think Meghna doubts Sandy, just like all the Chauhans she too believes that Sandy is the one who knows about and takes care of Karan. This is wrong on Naina's part to not let her sister know about Sandy.
4.What is a family? The one which doesn't let us go astray and keeps checking whether we're on the right path for happiness. Meghna is doing the same, she thinks Naina is spoiling her life, I think Naina has to voice her opinions now, she isn't able to as her opinions would hurt her mom and sister.
I understand both the sisters, each is right in their own place and the only solution for this is Karan getting cured as early as possible. He should stop taking it as his fate and should try harder to overcome his issues or maybe start taking care of his treatments on his own without being dependent on Sandy. He should man up and stop being a self-pitied child.
PS - I so wanted Meghna to tell Kunal that Sandy is the one to always inform her about the details of Karan's illness but she din't and it felt like Sandy is saved once again.