After seeing constant RRB x PPG shipping, I got annoyed and finally posted this article. You see, they can't be couples because they're related. There are 2 ways to look at it. Which way makes sense the most is personal preference.
Way 1: Mojo was Professor's pet and like a son, so in a way, Mojo and the PPG are siblings. The RRB are Mojo's sons, so the PPG are the RRB's aunts.
Way 2: Since Mojo pushed Professor and caused the Chemical X to pour in, he could be considered their father too. That would make the PPG and RRB siblings.
Normally someone prefers to look at it one over the other because they're both kinda disturbing and choose to acknowledge whichever is less disturbing to them. If you find both equally disturbing and legit, well, even though it's more awkward, it's more the reason that they're related and can't be couples.
Why PPNKG x PPG also doesn't work: The PPNKG are just the PPG from a parallel universe, so they're technically related to the RRB the same way the PPG are.
If you thought PPGZ x RRBZ would survive, I'm sorry to say, they did not either. Here's why: Mojo (Z) made the RRBZ from his DNA and the DNA of the PPGZ. I was gonna say that that made them half-siblings, but holy crap, I just realized something. Mojo+PPGZ=RRBZ Oh. My. God. The PPGZ are 8th grade mommies. Ya know, the writers should consider this stuff before publishing.
Well, there's my reason why PPG x RRB is wrong. Even if you don't ship Dexter x Blossom like I do, at least they're not related.