Better to rent a van and pay for the pickip and drop off fees. The trip from EIS to Nanny Cay for 6 totals $110 plus luggage fees. Then from Nanny Cay to Cane Garden Bay is another $79 each way. Then a couple of you will probably need to run in to Riteway for some last minute provisions. Another $30 each way. About $300 total plus tips.
You coud rent two cars for that price.

Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. - Mark Twain