You got to get a bit more understanding of the Multiverse thing when it comes to alternate counterparts. In this case with the Source and Firsts....
Most of the people we know in the Source probably are dead in the First or are turned into Sin Eaters since majority of the world is now covered by the Flood of Light while few we know are probably still alive or few we know are dead in the Source are alive. You can bet that they may have already planned for this and one of the Sin Eaters we meet and kill maybe someone we know in the Source but their First Counterpart ended up as a Sin Eater due to the FLood of light consuming their home region such as the First version of Ishgard.
The majority of people we meet are probably going to be the people who should be dead in the Source due to their different life choices and encounters, never met due to different life encounters and choices, or were originally never born in the Source.
It is just how Alternate Worlds/Realities that parallel our world works. The science itself to calculate what can and cannot happen is just way too unpredictable to know what X or Y can happen in a Parallel World to say it is impossible to encounter a version of some one that lived completely different life or not.