I see the Gary Russells 2 younger brothers are continuing their prospect status winning streak. The eldest isn't that old, so it is technically possible for all 3 to be champions at the same time. Would this be a first?
The Smith brothers could make it 4 brothers holding titles at the same time. But er no that's not going to happen. But to be fair although we like to criticise the Smith brothers, in particular Paul who kind of asks for it, it's pretty impressive that they've all held or fought for world titles (well Callum hasn't yet but will shortly).
I think the Kameda brothers kinda did it. Back in 2013; Daiki had the IBF 115, Tomoki had the WBO 118 and Koki had the WBA 118. But back then, Anselmo Moreno held the WBA Super 118lb belt... so, yeah.