I have seen a genealogy showing Thomas DeWitt m. Susannah Allen b. January 23, 1748/49, daughter of Erasmus Withers Allen."History of the Old Cheraws" by Bishop Alexander Gregg simply states "Thomas married and died young".
Children of this marriage are: Frances DeWitt who may have married her cousin Frederick DeWitt Sarah DeWitt who married a McCall (or maybe McColl)If you have information on the McColl line, I'd like to see it. William DeWitt Thomas DeWitt James DeWitt Allen DeWitt
Again, I don't have this genealogy proven, but some proofs of the relationships are given in an 1824 lawsuit: Equity File #70 which plainly states that William Dewitt's heirs at law were "Sarah McCall his sister Francis Dewitt your oratrixhis sister and William Dewitt and Alfred Dewitt the sons of his brother James Dewitt."
A lot of work is needed to fully prove this genealogy.