What to say about them? Brothers are like shield to their sisters no matter elder or younger.
Arjun did a mistake of stealing the paper. Big mistake. But no one in their wildest imagination would have thought that it could lead to Sam getting into a bet that too that would involve Naina. When Arjun came to know about the bet, he confronted Sam that he should have fought with him instead of placing a bet involving his sister. For any brother to take that his sister was manipulated and made to confess love is not a small thing. Even if no bet was involved the whole love thing would not have gone down well with Arjun.
How to expect a gal to stand for a guy in front of her family? Who wants Naina to confront Arjun? Like really? Inspite of knowing the whole bet fiasco, there was no direct talk between Naina and Arjun except Arjun asking tits and bits in anger. They do not share or probably no one shared the open talk repo with their siblings in those days. Who wants her to stand up and say that Sam was right, and her brother was wrong and to whom she should tell? She is the same gal who can't even request her dad to come to PTA where she would be given a prestigious award.
Also, a gal stands up against her family for a guy only when she is married to that guy. Whether there is a relationship between Naina and Sam or not, she will not be able to stand up and ask her Brother or Dad not to say anything to Sam. It is not about right or wrong, its about that time that era.
I have an elder brother. I had an affair in 90s, teenage love which dint go any further though I was very serious. The whole thing made me so reluctant to even develop a feeling of love that it took my now husband, who was my best friend then, 5 years to make me say yes to even start dating.