Paranoid my boyfriend likes my sister?

me and my boyfriend have been together for 2 years and known one another for 4 years. i have a twin, she's not identical& quite pretty.
whenever we meet, he always prefers to come round my house (my parents love him), and i'm scared that he may like my sister& that's why he always wants to come round. round the 1st year of dating we'd always go round his, but now we always go to mine & his excuse is 'i haven't cleaned my room'.
in general, he's a lovely guy. i know he cares about me deeply etc.
my sister always comes in my room when he's there and talks. they joke around and my boyfriend often insults her as a joke like calls her stupid. this often winds me up and makes me jealous so i tell her to go (i like spending time with him on my own, and she just sits and talks for ages).
the other day i asked my sister what she thought of my boyfriend and she said he was flirty. i then got mad and upset and she said 'oh no i meant he just jokes around a lot'.
i know they don't talk on text or anything, whenever she posts a picture he doesn't like it. but, we have a group chat on facebook with me, my boyfriend, my sister and another friend and i get jealous when she says anything in the chat as i know he will reply.
i don't know if i'm being paranoid. i can't honestly tell if he's flirting with my sister or just joking around.
e. g. in the group chat my sister accidentally sent a picture of our dog, and she said 'sorry i didn't mean to send that' and my boyfriend quickly replied 'can i forgive you? mmm, no'.
i know he jokes to his male friends and calls them names as a joke, and he does that with my sister. he's never said anything to me directly that made me think anything.
what do i do? i don't know if i'm going crazy.
im 18 by the way.
Paranoid my boyfriend likes my sister?
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