Finally watched the episode I might be in the minority, but I don't think that Rudra choose Shivaay over Om. He made his own choice after processing the situation the best he could with the limited information. He chose to go with his family not because of Shivaay, but because he wanted to. We all love the Omru and ShivRu moments, but at the end of the day he is his own person with his own thoughts and feelings. We have an idea about Shivkara's personalities and where they stand, but Rudra is still a complete mystery. This is one of those rare moments where he speaks his mind without relying on jokes (like the raksha bandhan episode when he confronted Shivaay).
It's no secret that he plays the mediator between the two, but clearly he wasn't in the mood to play that role in this situation. I find Rudra the most honest with his thoughts sometimes he can be a little too honest. Since the beginning he has commented on Shivaay's anger issues, his ego, and his inability to speak properly to others. That's what siblings do point out our shortcomings.
Although, the Ishana comment sounded harsh he was right Om does empathizes with others. Is that a bad thing? No of course not. It's one of the reason why most of us love his character so much. His ability to connect with others is endearing.
The problem is he biased against his father (and rightly so) and this distrust causes him to shut off automatically when he is involved. Rudra and Shivaay aren't exactly fond of their fathers either, but they haven't given up on them completely. They found a way to deal with them (Rudra jokes, Shivaay ignores). Despite his indifference to them Shivaay's trust amplifies when his family is involved. I think if in the future a conflict does arise we shouldn't chalk it up to him choosing a side, but rather him going with whatever he feels is right.
The little Rumya moment was a sweet surprise. She made him laugh and I loved it.