Why does he act jelous?

My husband always act weird whenever I bring up my younger sister. For example when I tell him I'm going out with he he gets all mad and talks so much crap about her. I can't stand it and she didn't do anything she is so nice to him. It's so annoying I resent him for this I don't get y he acts so threaten by her. Just because we are close. I always pictured my husband getting along with the closes people to ne but he is the exact opposite. What should I tell him my sister wants to go out for dinner Friday and I know he's gonna have alot to say but I font care I'm going. That's my family if u can't accept that I really feel we will not last