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Eris is Venus' step-sister. While Eris has an insurmountable respect for Venus and wants to emulate her, Venus had a complicated relationship with Eris as a child. Eris came into her life at a time where Gaiane was finally able to manage having two kids, and gave Venus the attention she needed, but was disrupted by a new baby coming into her life. She was extremely jealous of Eris, and some of those feelings still persist to this day - though it's not as bad as Venus' jealousy towards Earth. Venus, while she does love Eris otherwise, is not particularly attentive to Eris' life as Venus is focused on her own world.
Venus is Eris' oldest step-sibling. She looks up to and admires Venus, and much of Eris' "bad girl" tendencies are inspired by Venus herself. Like Earth, Eris also refuses to pull practical jokes on her, though for significantly different reasons; she just respects Venus that much, and... well, Venus' temper isn't exactly pretty.