Wow, Ron. So much supposing here.
She needs therapy. Stop trashing her sister. It sounds like the one stable person she has. I’m a skeptic, I admit, but I’m not fully convinced someone with zero social relationships has a fiance, is he IRL? Tell me more about him.
She said they went to the hobby together and the new friends starting texting her sister and “checking up on her, not me”
How is that the sister’s fault? I text people I vibe with. It’s clear there are some issues here, with a person who can’t make friends and hasn’t made any friends of note at all in fifteen years. You’re putting a lot of this on the sister and the fiancé. Imagine being someone’s sole social outlet, other than a sister she apparently also disdains. He’s already being most of her social outlet. It’s not on him to help her make friends.