Ok some 2.1 cards will work in a 1.0 slot and some not. It all depends if the bios is up dated. Even though the PC's are the same is bios is up to par and your's isn't. So I think that is the case.
Or your PCIe slot is broken or something wrong with the MOBO but that can't be because when you use the integrated GPU it works fine. So either it is what I stated above or the slot is broken.
We know it isn't the card because it works in your brothers rig. We know it isn't the PSU because both rigs run just fine. So it kinda of narrows it down a bit. Have you had any other card before this one?
You could try to up date your bios and up date the drivers in your system then try your card. One more question does your brother us a card in his rig? Get back to us with the answers. Good luck.