So much wrong with this situation.
Don't let anybody tell you how to run your life. **** em.
They are using the relationship to hold you hostage.
That's an adversarial relationship from the start.
If they can't deal with your happiness that's their problem and their loss.
They truly sound mentally unstable.
printcraft got it right on this one. It's your life. Don't hide yourself or what you're about. Your sister will get over it, and her husband sounds like a blowhard. Might as well rip the bandaid off, it's coming off one way or another.