No two individuals should be compared, its a known fact that Karan got his compassionate heart from his mom whom he names as his role-model while NK we don't clearly know what he went through in his life, many things we know about him are his current personality traits and our own assumptions...
Sandy is a heavy game planner and I wonder how a single person could manipulate the whole family, was it that easy!! I guess it was as the son who had the responsibility to hold the family swayed away easily in her game-plan as he was filled with resentment and insecurities... and then slowly came the downfall of the whole family, his wife who might've looked upto him and their toddlers... But still I give ot the credit to Nirmala and dadu for inhibiting such good values in both Kunal and Karan while Khyati became the spoilt-brat under her dad's guidance.
I don't think Sandhya has much grip on Kunal, she couldnt have stopped him from marrying Meghna, he was so in love with Meghna that he himself would've announced his love to the family if Sharda din't approach them. Sandy's grip on Karan ends with her care and his trust on her... these two people are the strongest characters in the Chauhan family, even though they love their maasi maa they both do know what is right from wrong.