We've stayed at Sebastians but we had a rental car. I wouldn't stay there without one unless you just plan on staying on site. Would highly recommend Sugar Mill for a dinner no matter where you stay - it's one of my favorite dinner places in the BVI's.
We're staying at an Airbnb rental in CGB in April this year for a few days pre-charter and not renting a car since everything there is within walking distance. Not sure when you're travelling but I'll report back here on our stay.