I'm not married, but have a friend that just went through a lot of family drama concerning her wedding. Finally, after over a year of stressing out about all of it, she just notified people that the ceremony would be on this day at this time and they would love for everyone to be there. And if you don't show up, that's fine too. She wasn't going to stress about people in the family not getting along. It was HER day. Although, she did not invite her step-mother. She was adamant about that. Her father came without her. Her wedding was amazing!!

Anyway, my point is, do what makes you and Josh happy. There's got to be some kind of compromise you can come to regarding his sister. I'd do as others suggested......invite her to the wedding (of course) but not have her in the party. If she doesn't "like" you then I can't see her being very shocked about not being included.