Yes, vitamin C will help but not prevent it. The ONLY way you won't get sick is to lock your brother away or move out and I know these are not options.

If you are really that desperate, wear a mask and practice really good hygiene. Be forwarned that if it's going to spread, it's going to spread. There's nothing you can do about. If you do happen to get sick, remember GRAVOL. Works like charm and you'll sleep your days away without the nausia.