Well, I don't see any way that doing this now won't end up putting your sister in a difficult position. Even if she has the strength to argue that she will keep you in the wedding, she shouldn't have to deal with any additional stress.
Is it possible for you to keep quiet until after the wedding? You can use the time to make plans.

We have all left home at some point. Occasionally we go back and then leave again (because it's so much better away from home). Really the most difficult thing about it is the worrying. It can be paralyzing.

The very best way to handle all of this is to stay calm and not argue. Your parents will see that their guilt, pleading, and threatening are not going to work. Just calmly and rationally say something along the lines of "I have decided that I want to be with (boyfriend). I really would appreciate your support with this. I will be moving out on (date)". If you decide to leave after the wedding, then don't tell them until it's over.

They will yell and argue and cry and do what parents are good at, but the trick is to stay calm and reply with a smile and a "thank you" and "don't worry". That will show your maturity, determination, and resolve (even if you don't feel it!).

They may or may not come around and it might take hours, days, months, or years. The important thing is that you continue to attempt a call and an email to say "hi", send those birthday and holiday cards, and keep asking for a lunch date with them. Doing these things will give you an advantage....and peace of mind.

Good luck to you and try not to worry. You're an adult