No one can really tell you if she was off course until we hear from the Captain and/or the crew, what the destination
actually was and the exact point at which they abandoned ship.
Until we know that it's all guessing.
That said, if they were heading for Bermuda they missed it by a country mile and they wouldn't be the first (or last) to do so
Perhaps their GPS
failed, again just guessing.
Everyone who has made those trips lucked out in one way or another and arrived safely, in spite of weather, bad seamanship, bad boat preparation, equipment
There have been several threads on this site about boats being abandoned as they made their way to Bermuda, so your sister's ship is not the first.