"Never argue with an idiot. They drag you down to their level and then beat you with experience"
Jason, if I were you I would have definately gone. First of all: it's only money. You can always make more but you can never re-visit the day of you sister's wedding. Second of all: it was obviously important to your sister that you be there. She's not (in all fairness) asking you to live you life a certain way for her. She's asking you to be there on what she considers to be one of the most important days of her life. That's all...just be there. And surely it would have made you happy to make her happy, on some level?
And, as you say, whilst your family may be angry at you at the moment, they accepted your decision. They didn't try and force you (at least from what I can gather from your post). Does that not further demonstrate that they were not trying to stifle your happiness (btw I think you're being a tad melodramatic by saying your happiness hinged on whether you went or not) but that they genuinely want you there?
Just an observation man, obviously I don't have the full story...
"If a man could pass through Paradise in a dream,
and have a flower presented to him as a pledge that his soul had really been there,
and if he found that flower in his hand when he awoke - Aye, what then?"
Samuel Taylor Coleridge 1772-1834