LOL @ Shivaay thinking Om wants this marriage when his hate is so visible, but the writers need him to misunderstand their relationship so she can stay. His words to Om were he hide his happiness from him, which implies that he thinks Om hide his marriage not that he doesn't accept the marriage. Om had several chances to his brothers, but he didn't ...again another plot device to move the plot along.
How Shivaay found out and what he knows seems vague. The writers used him to bring her back and will use Daadi to make her stay.
Honestly, I felt bad for ShivRu then and now Om for being emotional blackmailed into doing things they don't want in the name of family. I hate how much they interfere, but we wouldn't have a story without it 😆 Omru did it for Shivaay and now its ShivRu/Daadi/Jhanvi/Bua Maa's turn to help move the story along for Rikara. I think Tej and Bua Maa (when she turns evil) will be the villain in their story because as a whole the character Jhanvi was never cruel.