I don't have any info on John's siblings either, however I do have siblings for his father William.
David Alexander born-abt 1737 died-Oct. 29, 1792 married- Anna Ewing Samuel Alexander born-abt 1743 John Alexander born-abt 1745
There is a chance that Bethiah was the child of one of these brothers. I will have to check into this further. I have never really researched the children of William's brothers maybe I will try looking into that further. You never know. Thank you for your reply. Joyce Davis ----- firstname.lastname@example.org