Ammaji is in pain because of someone else's happiness not because of something of her own loss.
Weather Radhika marries Dev or not, it does not change Amma's life but her jealousy is killing her seeing Radhika's happiness.
I always wonder, why people instead of gaining happiness by bringing joy to their life, bring pain by trying to ruin someone else's happiness. This is more painful than any other pain.
Jealousy is a slow poison that kills the calm in one's life. Instead of their betterment, they try to gain happiness by harming the other person.
Because of her jealousy, she is demeaning her character and there is one more entry in her account of bad karma.
She is trying her best to ruin the wedding but whatever she does, finally whatever is in Radhika's destiny will happen and that is already decided by God Almighty.