After seeing constant RRB x PPG shipping, I got annoyed and finally 投稿されました this article. あなた 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...