They're just retconning the PPG universe - Now, the girls weren't the first experiment the professor did to create the perfect little girl. His fist attempt was now Bliss, which he did years ago which is why she is older. For whatever reason, his formula was incorrect (Something along the lines of using Sugar and Spice, but leaving out everything Nice)
causing her to be a bit of an uncontrollable Super-Powered Brat. Chemical X WAS used in her creation and that's why the Professor wasn't intending on using it on the new PPGs, since if the new girl came out bratty, at least they wouldn't be super powered and he'd be able to raise them normally.
They're going to hand wave all of this as Bliss running away, and the Professor thought that was the best thing, completely ignoring the child endangerment, and that he never told the girls they had a sister because it was too heart breaking to talk about. Bliss comes back because she hears of the PPGs and is jealous the Professor has new kids, so she's going to befriend them and then try to take revenge. The Professor is hesitant to have her back in their lives, but the girls assure him if she gets out of control, they'll put her back in line.
The creators are going to try and talk about how it's progressive because they're dealing with matters of toxic family life, abandonment, and rehabilitation, and so forth - But people are just going to rag on it because it's moral is confused and heavy-handed without actually meaning anything, it's not well written, the character of Bliss was made just to pander to a particular audience, and that particular audience is going to be mad the black girl was both abandoned and made the villain.