Not necessarily, but it could be fleshed out a bit more for the purposes of not drawing too many parallels with this episode and "One Bad Apple." Make her *very* sneakily evil
If it's going to be a cousin, why not make it a bit older Rarity's age, if she isn't already? (I kind of got the impression she was Sweetie Belle's age ) and why not make it someone Rarity greatly admires for her craftsmanship and artistic ability, as well as refined manners, to the point of having looked up to her greatly when she was young?
Maybe she's why Rarity seeks to be refined? She could even mimic the beatnik Rarity style a bit
However, she'd also be the anti-thesis of Rarity because she'd be snotty, unaccepting, and greedy. She'd be what Rarity haters accuse the goddess of fabulosity herself of being - and it'd shine through when she tricks Sweetie Belle.
So not only would that help provide insight on Rarity's true character and what she *could* be, but it'd provide a moral dilemma and test Rarity's good nature - go against the mare she looked up to for almost her whole life, or get after Sweetie Belle for directly stealing money for her? What to choose?
That's off the top of my head, though