Oopsie Starlight and Stardust Sparkle are related, most likely sisters or cousins.
They literally look so similar that this troper literally thought Stardust was just an updated Oopsie when they first saw her.- The animated music video
more or less establishes Stardust as Oopsie's apparent replacement, though this guess still isn't entirely unlikely.
Poopsie is an In-Universe franchise in Rainbow High.
One of the main characters in the Rainbow High web-series, Sunny Madison, has a Poopsie character on her phone case. The Fantasy Friends characters also appeared briefly as "Hi-Hi Kawaii" characters, albeit with some tweaks to their designs. It's possible in the Rainbow High universe, Poopsie is a character brand comparable to LINE, Mickey Mouse & Friends and Sanrio.- Alternately perhaps the Poopsie characters are mascots Sunny developed for her project, since Hi-Hi Kawaii is most likely her creation (unless it is an existing franchise, and the characters shown were In-Universe Self-Insert Original Characters.)
The reasoning for Poopsie's cancellation.
To the sorrow of Poopsie fans (and to the relief of... everyone else...), the Poopsie brand has been canceled for 2021, and there might not be any more releases under the brand. Clearly, the backlash over the Nausea Fuel-inducing elements of the toys is not the ultimate reason why the toyline got shelved, as it lived two years. However it’s peculiar that the toyline was cancelled when it hasn’t shown signs of Seasonal Rot in later 2020 releases.- Most obvious case is the overwhelming success of Rainbow High, which was developed from Poopsie's Rainbow Surprise spinoff. The dolls are mostly warmly-received and the tie-in webisodes are fairly popular, so MGA saw fit and pulled the plug from Poopsie.
- Lalaloopsy managed to make a comeback on at least some of MGA's merchandise at the same time Poopsie stopped production, so that could also have a hand in the line's demise.
- Poopsie had also gotten MGA into at least two legal troubles regarding it primarily relying on parodic content, so it's most likely the company didn't want to take any more risks.
The Rainbow Surprise Girls were meant to be AU humanizations/counterparts of the Surprise Unicorns a la My Little Pony: Equestria Girls.
I dunno, this idea was in my head for a while.- Rainbow Brightstar -> Rainbow Dream
- Oopsie Starlight -> Pixie Rose
- Whoopsie Doodle -> Amethyst Rae
- Blue Skye -> Dazzle Darling
