Responses
I also think his brother was there, but I don't think he wanted to help; on the contrary, based on the deep hatred his brother has so far manifested against Siwon, I would argue that he is the mastermind of the rape. He probably paid those thugs to rape Siwon.
if i remember correctly his brother said something like you gonna beg me again for my help if you continue to act like this
if he raped i think his brother was there, he just sat and watched he wanted to help though