I'm convinced that Yamini is crazy, she is so abnormal that even her husband can't deal with her these days. in addition, I suspect she has some hidden motives as well. otherwise, her fears were are very real because a man was attacked in the engagement, Viren is on the brink of death, and Rithik almost died. Besides, Yamini knows she and her husband are hiding skeletons in their closet- secrets that consist of cold blooded murders. so they know a naagin is after them-particularly Rithik, then of course there is the rithik lookalike.
The circumstances were such that Yamini had no choice but to agree. she does not like shivanya or trust her but there was only 10 minutes left and she wants to save her son's life, so this was better than not getting him married at all.
As for shivanya, Revenge is only thing on her mind as of now. she will do anything to avenge her parents' murder. the reality of this marriage has not sunk in yet, and I figure in a couple of days she will realize what she has gotten into.