You could try slots 2&4, see if that makes any difference.
Have you tried both sticks individually in both slots?
Try enabling XMP whist you have it in your brothers rig, just to make sure the profile is working.
While you are at it, try his ram in yours and see if you still get the same problems.
Sorry mate, looks like you'll have to stick with a working speed, I doubt you would have noticed difference at 1600MHz anyway![]()