Two years for me. On my wedding day, six months after, not one of them made contact. They wouldn't have been struck by lightning just sending a text message. From that day on, I stopped missing them. I was having such a great day and they didn't even nod.
My point is that if I do hear from them one day, and an apology isn't within the first few sentences; not for the wedding thing, just for going along with the shunning... I don't think I'd want to continue the conversation. I would, just in case they're in a less stable situation than you re whether they want to stay witnesses, but I wouldn't want them back in my life if they don't acknowledge the way they have treated me. You might be surprised at exactly how badly you have treated them.
I think your sibling would understand completely if you told them that you're very subtly trying to make the move out, and would not sabotage you. You'd know better though; maybe your sibling isn't so bright.