From the coroner's point of view your brother may have willingly taken the drugs, not to kill himself but to get high, and however much the family may protest that he doesn't take drugs etc, they've heard it all before, and quite often families don't know everything about each other's lives. If that is the case then the most the other men can be charged with is supplying the drugs, if there is evidence to support this, or they confess. The fact that someone else died in the flat of a drug overdose simply indicates that users frequent the place, not that murderers do. I know it must be very difficult for you to accept that your brother has died in this manner, leaving so many unanswered questions, but the best thing you can do is try and move on, because you will never be able to conclusively prove anything, and you will only make yourself ill with the stress of trying.