@onedayillbeamillionairemaybe This is a tricky one.
On the one hand, it seems harsh to let your friend down, but on the other hand, this is very close family. I am sure that your sister hasn't done this on purpose, but then again, she seems very dismissive of your issue.
I know a few people who have not gone to a siblings wedding, and chosen a holiday or a mate's party, or something, and the relationship with the sibling never recovered.
In addition, several people I know still have a bee in their bonnet about the fact their brother or sister didn't attend their wedding, which was 25 to 30 years ago. It can leave a permanent rift in a family relationship.
So the ball is in your court really. If you care about/love your sister, and you have a decent relationship, you should choose her and her wedding over your friend. If you are indifferent towards her, and don't give a shit about your relationship with her, then pick your friend instead.
Friends come and go, family is forever. Whether we like it or not. And as I said, if you ditch your sister, expect it to never be dropped by your family, that you let your sister down. I am not saying you will be letting her down, but it will be seen by your family, that you let your sister down.
It's your call.
That said, it's looking more and more likely that your really expensive non-refundable holiday that you've been saving for, for 4 years, may not go ahead anyway, with the way things are going in the world right now.