Don't get me wrong.... i think the main reason amma is lying is because of Radhika not Vishaka. Radhika has showed amma that all mistakes she does she will forgive her easily and if god punish her also she will fight with Kahna and save her. remember after the weeding during her biddai she prayed to Kahna to heal amma even though she was unable to that... this shows that whatever happen Radhika will still help amma, i think that the main reason she took the dare and promised upon kahna and not her son because she knows if she promises on her son no one can save her son....