Just a quick response...
from my experience St. Kitts is not friendly to sailboats,
and their Customs and Immigration can be a nightmare for small boats. Plan on dealing with them taking two days!
Customs and Immigration is two different departments, and when one is there the other one usually isn't, so you have to come back the next day! (Unless you are a Cruise Ship)
This can also complicate things for Nevis.
If you go to Nevis, they probably will make you return to St. Kitts to check out, and that can take another two days!

On a better note, spend a night in Columbier in St. Barths, a special place usually only seen by boaters; it is a good snorkeling spot.