Hi Susan,
Looks like this needs some proper research.
At present the records suggest that David & James were twins as they were both born in 1706 to David Wemyss & Elizabeth Lock(e) and that Williamina was born on 6th. Dec,1704.
There is evidence to suggest that Elizabeth Lock was really married to a William McDuff in 1705.
David died 5th. May 1779 in Anne Arundel, Maryland
James died 13 Aug 1781 in Calvert, Maryland
Williamina died 1784 (locus unknown)
Does this help?
Graeme Deighton-Hastie
Edinburgh
Scotland