No evidence to back this up, but I suspect that couples who decide to marry abroad - especially in some paradise island beach wedding - are more likely to become divorced within a few years. The whole dream wedding abroad scenario smells like over-compensation for deficiencies in their relationship. Plus, as dear zooo alludes to, it's a bloody selfish way to get married. It's miserable and tedious enough having to attend family weddings without having to pay a small fortune for the "privelege".
Anyway, all the best for your sister-in-law and her fiance.