Excluding the fact they're real life cousins it's possible they're related even if CJ wasn't directly involved with him. IE a child of a prostitute or a one night stand CJ had.
Yes yes we could argue about the different continuities but the fact they're Easter eggs, achievements, and the fact Grove street still exists. Makes you question whether or not they will bring it all back together in VI. Which will likely take place on the eastern side of the country. Whether that's Vice City or Liberty City or something completely new we'll find out. Chicago would be an interesting setting for example if it's new.
I think more people would buy the continuity difference if Grove Street or Ballas didn't exist or were a different named African American gang, I guess the only big difference is how San Andreas/Southern San Andreas is different then 3d Universe. But obviously there would be no way for those games to compete with current gaming in terms of resources and what you could do back then till now.
I think this video explains the Ballas well and makes since given the changes we see, and a partial history to the Ballas we see in V https://www.youtube.com/watch?v=WCSqJLI2Now
Either way I still enjoy the Classic GTA III, Vice City, and San Andreas after the debacle of the GTA Trilogy Definitive Edition.
IV to me was a good step in the right direction as you do need to evolve the game over an expansive franchise including those older 2d versions. V was also great I like how you can play as multiple characters which really gives you variety.
Also personally could give a damn on the race of a main character you play. As long as the story is decent and evolves that's all that matters. Perhaps VI could cap the end of the HD Universe as a Trilogy but it would need to connect IV, V, and obviously VI if they go that route.