Oh boy... okay...
These hood wearing hippo looking dudes are in the weapon shop in Alexandria the second time you visit (Disc 3, I believe)
The concept is simple, pick the correct brother.. but yeah... no human eye can keep up with the UNREAL SPEED these guys move at so, here's what to do.
Grab your phone or tablet, record a video of the shuffle then using slowmotion (I downloaded an app for this) slow the video down to 10% speed and watch it as many times as you want until you are certain. Rinse and repeat.
It's tedious, but hey, we've all done worse for some Gamerscore