First of all, this is a kinda common trope in anime but the artist manages to use these 3 characters to create the fantasy they are actually a caring family by looking at the ages and behaviors of the original characters.
-Jessie is perhaps younger than Frankie depending on the source, but she looks older so even if seems like she is a mother, she's more like the oldest sister in that family. Not only that but she can be as caring as Frankie just like in this picture, or as evil as Vicky.
We've also seen her using that outfit in Pokemon.
-Frankie is the one that looks more natural in this picture since she is 22YO so it would be common in Japan if she was already working in a company, that's the reason for her outfit, but the artist wanted to make emphasize in her wild but slightly mature nature by drawing her piercings and a can of coffee. The fact that she was the one that made the bento (Jessie can't cook) makes this better since she was pretty much in charge in her show.
-Vicky is 16YO so depending on the school, she could be wearing a uniform similar to the one she wears but since she's too aggressive it would make sense if she wears her uniform as a delincuent. She even has one of those anime brooms that are commonly associated with delinquents that are forced to clean the school after they got caught by the teaches. She also carries that broom as if it was a wooden sword.
On top of that, the artist was aware of the tsundere side of Vicky.

I just wanna fuck a family of crazy red heads.