Where does it say the sister “walked off” with the friends LW was starting to make? Or that LW was having success pre-sister? She wrote that the new people she’s met will text her sister, but not her. And that pre-sister, she felt like she was coming off too strong in her hobby classes (i.e., not having success). She also writes she hasn’t confided in her family about how she feels about the state of her social life, so why would they think to help her? The extrovert sister is the primary person against whom LW compares herself because it seems like there’s nobody else in LW’s life… her way of managing her insecurities was to push everyone else away.
The wedding stuff is a separate issue, IMO, and yeah, she should talk to her fiance about what’s going on with her (there’s no way he can’t know by now that she has no close friends, he probably thinks she’s content), but I’m with @ktfran that the onus shouldn’t be on her fiance to solve her problems. Nor do I think he could even if it were his job because what actually needs to be done is inner work. Expecting your partner to fill all these different roles in your life seems like something more straight men than women do. I’ve definitely dated men who seemed to need a therapist or cruise director to fill their social calendars, but instead they found a girlfriend.