My boyfriends 'sister' isn't related to him--he forgot to mention.

I've been with a seemingly incredible man for around 5 months now. Every moment has been filled with open communication. He's nearly a decade older than me, in his 30s, and he LIVES with what I thought was his sister, and her boyfriend. He's referred to her has his sister EVERY single time we've talked (my sister is super protective of me, my sister calms me down, I don't know if I could move out with my sister, etc), to the point where I've joked that he doesn't need to keep calling her that, I know her name. He's had a traumatic childhood, so while I thought the sister obsession was a little odd, I forgave it as shared trauma.
Fast forward to yesterday, where he casually mentions that they met '10 or 12' years ago playing video games. I was understandably confused and asked for clarifications and he looked me dead in the face and said 'we're uh....I guess adopted siblings'? He would have been at LEAST 23 when he met her, she's around the same age as him.
He offered no more clarifications and when I tried to open up the topic for discussion as my heart lay on the floor, he kind of shut down and wasn't open to it. No information about when they met, if there were feelings, past intimacy, NOTHING. And when I jokingly said 'I guess it's good I'm not the jealous type' he made the most disgusted face and snapped "Thats my SISTER". I've had best friends for 15+ years. I've had roommates. I've had people I grew up with since I was 10. But none of these things would I ever feel comfortable just announcing to a romantic partner as a 'sibling' and leaving it as that. He made me feel absolutely insane for even questioning this made up relationship, meanwhile I felt utterly lied to and like a complete fool. After he shut down I got panicked and made a few unrelated jokes until *he* felt ok again, like he wasn't the one that dropped that bomb on me. Am I insane?? How do I address this??
OH. And the biggest red flag, he hasn't dated in a long time before meeting me...around 10 years, right when he met her.
Tl;dr: My boyfriend failed to mention the woman he lives with isn't his bio sibling, and seemed shocked that that mattered to me.
​


Source.

Maybe he should of said she's a NUN then tell you she's his Sister no that makes too much sense