Cousin, sister, father, mother, best friend...if you can't go, YOU CAN'T GO. It could be for anything; you're sick, your work won't let you, you have other obligations, you simply can't afford the trip, etc. Hell, even if you flat out don't feel like going, oh well, your right. I can tell you right now that if my best friend of 24 years was to get married in the next 4 months, I would try, of COURSE, but right now? I couldn't do it financially at all. Just not happening.
If you feel like going, go. If you don't, don't. Send a nice card and/or gift and call it a day.