Wow, I was not expecting to see this response here.
The people lashing out saying stuff like "brides think the world should revolve around their wedding" etc, are very harsh in my opinion. Her sister has had to reschedule her wedding, and no doubt have to come to the realisation that the day she does have will be very different to the original plan. I don't imagine she was in the right head space to ask every important family member their schedule for the next year or so.
I do think as her sister and MOH, OP should have been there, helping her through rescheduling and therefore letting her know the date she couldn't do, before it was planned. This could just a massive communication error.
I personally wouldn't miss my sisters wedding for the world, not a chance. No holiday, expensive or not, would take that experience away from me. Those saying "a weddings only an hour anyway!", that's not the point. The point is being there for your sister on one of the most important, and happiest day of her life. Well, so I think anyway. But each to their own. If I was the friend in this situation i wouldn't for a second expect you to still be going on the holiday with me. I honestly think if I was on holiday, and I knew my sister was getting married back home I wouldn't be able to relax. So might be best for think about how you'd feel while away, if it'll ruin the trip there's no point is there. But if you're happy to let it go and enjoy yourself, then so be it.
this is from someone who found out her brother had booked his wedding for a time when i would be on holiday in Asia. The thought of missing the wedding made me feel so upset, the holiday instantly didn't matter. I didn't give a toss about it when it came to it. And before this I'd been desperately looking forward to the holiday, and same as you, it was an extravagant holiday that really would have been once in a lifetime. But I saved up once, so I can do it again.
It's a crappy situation, it really is and I'm sorry you're in this position! But this entire year has just been a run of crappy situations and difficult decisions for people, just got to do what's right for you and then live with it.