If you do a search on the topic "rain" or "weather" you will find this question asked and answered over and over again. You will also find that I tend to answer this question by saying "It only rains on Thursdays."
The points are:
1. Weather forcasting is inaccurate at the best of times
2. Chances are that in the Caribbean you will get a lot of sun, some clouds, some wind and a little rain.
3. Whatever the forecast, your tickets are bought and paid for and you can't change them.
4. Don't worry about the ******* weather! You WILL have a great time.