Note: " The book " History and Genealogy of The Harlan Family, Chester County, PA 1687" states that Allen Langley is "most probably the son of Jonathan Langley of Christianna Hundred, New Castle County, Delaware."
Many accept that as proof, but I've never been comfortable with only that bit of text. Allen did name a son Jonathan, my line, so all of the pieces are there. I recently hired a genealogist in Delaware to get to the bottom of this. It was well worth the money and I now have proof, probate records, and Allen has verified siblings as well!!
Jonathan Langley married Sarah Mendenhall about 1740- 45 in Chester County, PA (not proven)
Children (in order as listed in the probate document)
John Langley Allen Langley Thomas Langley Curtis Langley Rebecca Langley Mary Langley" ( Rich Langley Kokomo, Indiana) 
" Jonathan Langley Sex: M
Birth: ABT 1720 in Christiana Hundred, New Castle Co., DE
Death: in Christiana Hundred, New Castle Co., DE
Father: Dennis Langley b: 1690 in New Castle Co., DE
Mother: Jennifer Hamm b: 1700
Marriage 1 Sarah Mendenhall b: 1723 in East Caln, Chester Co., PA
Allen Langley b: 1739 in Christiana Hundred, New Castle Co., DE
Thomas Langley b: ABT 1748
Curtis Langley b: ABT 1750
Rebecca Langley b: ABT 1751 in Christiana Hundred, New Castle Co., DE
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