AntiEverything, I can see that it is a bit difficult if your sister's wedding plans are so extravagant that they will exclude important guests and family members. After all, the wedding isn't just for the bride and groom, but also for those who care about them.
However, I must say that the opposite extreme also has me disagreeing. I certainly agree that you shouldn't HAVE to prove your love by providing what is more like 4 or 5 figure wedding rings and overseas trips. However, I think that there is a lot to be said for giving because you want to give. I wouldn't love someone just because they do special things for me, but when you love someone you often WANT to do those special things. I think it's all about finding the middle ground and getting a balance. If I ever get married, I don't want a majour fancy affair: I would want a nice dress (nice does not have to mean $2300), some nice flowers, and a nice reception (something like everyone hiking off to a nice outdoor place and munching tasty treats like scones and jam!). Receptions are the killers, but personally I think the idea of a 3 course meal is rediculous! Save bringing out the fancy shtuffs for the honeymoon when you only need to pay for two people! 
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...though fire may burn & flames envelop me, I will arise from the ashes...