Why do you assume the sister is intruding? I actually assumed LW invited her sister. As an introvert, I often prefer a friendly face around and if I want to do something new but feel nervous, will ask a pal. Or my sister. How would the sister even know which class to sign up for if LW hadn’t mentioned specifics?
As kids, sure, maybe it’s important for siblings to have different activities to individuate, but it seems stupid that two grown-ass sisters couldn’t go to the same ceramics class or whatever without it turning into sibling rivalry.
Not to mention, at best, the sister is acquaintances with these new people. But LW has decided, because she is still ruled by her insecurities, that they perceive her sister as better. That she is lesser. That there’s something fundamentally wrong with her.