It is karishma who likes to see tamasha. Mohit is just mute. When you are the "khota sikka", when no one ever takes a stand for you including your oen parents and you have 2 older brothers who are the apple of everyone's eyes, one can easily get caught up in bitterness. He probably looks up to his older brothers but also feels jealous/bitter/inferior of the attention and laad/pyaar they get. I don't recall seeing any scene where mohit has deliberately hurt someone but he has never spoken up because he has no izzat in this house, every time he has tried he is shot down in 2 seconds. Even devi/ashwini/shivani can put up a fight longer than him. Hes the youngest son who in most households is the pampered one. But here he is treated as a nobody.
Virat knows right from wrong but he has been so conditioned that he really pi ks his battles. Instead of picking a side he usually just diffuses the situation pleasing everyone. Today was one of the few times like Sai's education where he has clearly picked a side and spoke up to everyone.
I feel that he spoke up for Sai's sake and her correct stance more than mohit. If Sai had not raised hell, sab log chhup chaap apna kaam karte, mohit would have left hungry, virat wouldnt know anything- this is probably what always happens in the house. Virat probably has never realised just how badly aai/shivani/mohit have all been treated because by the time he enters the situation either everyone has accepted their fate and moved on.Virat looks shocked everyday to see his family elders in a new light exposing their double standards. It is only thanks to Sai and the fact that she wont back off that virat is seeing more of what really goes on in this house. And when Sai starts her morcha, virat again usually speaks in a way to calm everyone down without blaming/pointing fingers etc but sometimes the issue is such that virat is forved to take a stance. To his credit, he does the right thing. But today was one of those few rare moments where Virat even though he came with the attitude of "kya hua hai, koi mujhe batayega" once he saw the truth and realised the reason for Sai's morcha, he was firmly on Sai's side and has not tried to just calm people down but actually shown everyone a mirror and given it back. To sonali, bk, ninad, pp. Everyone got a sharp rebuttal.
But if this scene had played out in front of Virat with no Sai in the picture, how would be react? Would he oppose it? Yes. But would he be as fierce, as blunt in his opposition as Sai is- i think not. He probably would calmly be like "kaku, khaana nahi khayega toh interview kaise dega" or maybe he'd just give a disapproving nod, take the plate to mohits room and tell him to eat. Or maybe he'd even say - ye thik nahi kiya aapne. But he would likely not be as fierce as Sai. She used all the right words. Dohraapan, "bheek manga", "adbhut parivaar". And it was important because she wasnt having a polite disagreement she was calling out their hypocrisy, double standards and the despicable treatment being meted out to a family member. Sai's fierceness enabled virat to also back her up solidly.
Virat ka "fierce roop"/insaafpasand attitude sirf bahaar criminals ke saath dikha hai, GC mein, but aaj for a change ghat mein dikha. And being able to call out and fight with your own family is much harder to do but its important. Kyunki safai karna hai toh pahle apne ghar se shuru honi chahiye!
Virat has always been righteous but until Sai came in the picture he lacked the "fight". He has mostly picked the overly diplomatic route which may have worked but sometimes it dilutes the impact. Similarly sai is always too blunt. She needs to realise that somerimes tact and diplomacy also work, work better than her approach. But situation ke hisaab se navra-baiko need to use each others ammunition.