I would suggest carrying out an insulation resistance test on the whole installation. A damaged cable anywhere could be contributing to this situation. I once had a situation where an outside soil stack pipe became 'live', and the builder was getting 'shocks' off of it. Turned out to be a lighting cable in the loft had been chewed by squirrels! Remember that electric shock of as little as 2-4mA can be perceived, 20mA is pretty painful and 50mA can kill, so your brother may well be receiving just a few milliamps. This would also explain why an RCD may not be operating. It also suggests that possibly, it is not the full voltage that is present. Have a look in the loft or wherever the water feed to the shower is, and maybe also the drain from the shower. Test any metalwork to the MET using an R2 lead (or an odd bit of singles) for voltage, and then continuity. If it is not coming off the metalwork, try the walls/floor etc. You may have a damaged cable within the building fabric that is 'livening up' the surfaces, especially when wet? Good luck, and stay safe!
