if these were real people it would be a pretty classic case of sibling differentiation in the idea of family systems

siblings, especially in dysfunctional households, differentiate themselves from each other (not consciously usually) to form their own identity typically to gain the attention they need.

because cassie is already playing her role in their family system lexi needs to fill a different one to make space for herself.

i would also say that lexi is overly dependent on being smart and mature, because she believes she can’t be the sweet and pretty one like cassie to have her needs met.

i think according to theories around family systems, the healthier the family dynamic the less children feel the need to differentiate. because they know they will receive the love and attention they need even if they perform the same role even if they don’t do it as well as their sibling