Each airport has minimum connecting times for on-line and inter-line connections. On-line is if you're connecting on the same airline and the latter is if you're switching airlines. Staying with the same airline usually has a shorter minimum then if you're switching. Staying on-line usually means you're flying with the airline who has a hub in your Customs/Connecting city; i.e. Delta in Atlanta, USAirways in Charlotte and American in Miami.
With that in mind, no one will sell you a through-ticket that does not meet the minimum connecting times. Computers won't let it happen. But in some cases you can save money by buying two separate tickets...if that is what you're trying to do....and of course you have to make sure you meet the minimum connecting time on your own. Call any
travel agent and they can look up the minimums for you on their CRS; the airlines can do the same for their hub cities.
I think 1-1/2 hours is risky for an international connection but if you're on a through ticket and it meets the minimum, the airline that sold you the ticket has to take care of you if they cause you to miss THEIR connection. If you're traveling on two separate tickets then you're generally on your own, even if you meet the minimum, because each ticket's job was to get you to the next place and the first ticket got you there, even if they were late, and you simply did not show up for the second flight on time.
Flights from both St. Lucia and Antigua use Customs at the U.S. airport, not before you depart their islands, as far as I recall. Also consider connecting in San Juan, where American operates a large hub between the islands and the U.S. mainland.
BobK/Orlando