It’s explained a little in GZ's mutant path, but it’s also explained in one of the paths in SSS, specifically the path where Suzy meets the couple from Love SICK. Lilith explains it quite a bit.
Basically the GZ Corporation (In a joint effort with the government) dumped their drugs into the water supplies in various test cities and observed the results (Or in Suzy’s case her small town) It was supposed to make the average human more resilient to the effects of radiation and basically make them better able to survive the inevitable nuclear apocalypse.
Of course what ended up happening is it tended to make most folks more violent than usual to the point where they became little more than cannibalistic animals. (They were never actual zombies though, since they're still alive)
However, the effect doesn’t last forever as can be seen in the mutant path in GZ. Eventually those negative effects wear off in most and they emerge transformed (mutated).
Also not everyone was affected negatively by the drug. Suzy herself for example wasn’t affected negatively and just gained the benefits. (She is able to thrive very well in a nuclear wasteland in many epilogues after all)
And yes, Leslie and Lilith are Bobby’s uncle and aunt.
