Hi there - We got married at Tensing Pen in Negril, Jamaica, and it was fantastic. Eight years later our friends and family are still calling it the best wedding ever. It was not expensive or stressful at all; we rented out the entire resort, and the only thing people had to pay for was their airfare. I think the whole thing - including rooms, wedding, and reception - was under $10,000. Totally low-key, no planning involved (they used local flowers, local minister, local band, etc. and planned it all - I'm not picky). We flew into Montego Bay, and Tensing Pen arranged for shuttles; it was about a 1.5-hour drive to the resort, maybe $20 a person? In any event, it's not a fancy beach resort - it's a rustic place out on the cliffs (right on the water) with a small outdoor restaurant and bar. No TVs or phones (although they do have WiFi now). We had about 35 guests, short wedding with a live saxophone, dinner and a band playing reggae for dancing. So, so fun - we just went back for our anniversary with some friends who came to our wedding, and it was like spring break for adults. If your sister is more into the all-inclusive/beach thing, I think there's also a Sandals or Beaches in Negril. Good luck!
Added: We went to Anse Chastenet in St. Lucia for our honeymoon. Also great, but not as secluded and rustic as Tensing Pen, and harder to get to.