<human>: Can you provide a brief rundown of the text in within 3 sentences?

This is a throwaway, because I'd die if he ever knew I wondered about this. My boyfriend is extremely close with his sister, who's 22 years old. For some reason I get an incestuous vibe from the two of them. Don't get me wrong - I'm pretty sure that nothing's ever happened, but I find myself getting extremely annoyed whenever they're together. 

It's tough to put into words, but here are a few things:

1. When they're together, it feels like they're speaking in their own language which leaves me out completely. They just talk in references and inside jokes which drives me crazy.

2. Whenever I lightly tease my SO around his sister, she always immediately defends him, as if I'm actually making fun of him. 

3. My SO always talks about how his sister is a genius, and how she's probably the smartest person I'll ever meet. She apparently got an 2400 on her SAT. (Who even talks about that after college?)

I am also extremely close with my brother, but he's 19 and therefore I feel like the age gap naturally lends itself to some healthy distance. I'm adding this just to say that I understand close relationships with siblings - I just don't get why they have to act like they're the only two people in the world when they get together. 

I also want to add that I'm pretty sure this is some weird form of jealousy. I never feel jealous when my SO is around other women, I know that he's completely loyal. But I guess his sister is the one person who is more emotionally close with him than I am, and that annoys me. I just want to know if anyone else feels like this, and if so, what they've done to remedy it!
<bot>: SO is extremely close with his sister, it weirds me out and makes me kind of jealous. What should I do?