I would check two sources for information on the Caribbean origins of your grandmother and her brother:

The World Genweb Project: www.worldgenweb.org/ There is a Caribbean Genweb portion of this site (www.rootsweb.com/~caribgw/) with links specific to several of the islands including the ones you mention.

The Church of Latter Day Saints (Mormon Church) www.familysearch.org/
has a link to its International Genealogical Index. You may search by name, event, year and region.

Good luck,
Alva Stevenson