UA is the only airline flying nonstop to SXM out of EWR. If UA is willing to accommodate you on another airline then that will require at least one additional connection to catch one of the few SXM nonstops (from CLT, MIA, or FLL), or getting yourself (and your luggage) from EWR to JFK in time to catch one of the SXM nonstop flights from there.
What may be more practical is to look at a connection through San Juan as a backup. UA has an interline agreement with Seaborne Airlines, a regional carrier which operates between SJU and SXM. Check flights for your dates to see if that might be an option for you.
Alternatively, see if you can change your flight to fly from home to EWR the night before, and find a hotel near the airport.
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