Why is Abraham Lincoln a founding father?
He is not a Founding Father.
Technically, no. Lincoln was not a Founder. Rather, the Founding Fathers were those were present for the making of the Declaration of Independence and the Constitution, and those who shaped the country as it was being formed. Lincoln's legacy in history is as the Savior and Protector of the Union, but not as a Founding Father.
Nancy Lincoln gave birth to Sarah Lincoln shortly after marring Thomas Lincoln. The father of Sarah is unknown. Thomas Lincoln was castrated when he was 12 and was not the father of Sarah or Abraham. Sarah and Abraham were brother and sister and they were not married. Abraham and Sarah had the same mother, but different fathers. Abraham's father was Samuel Davis. Abraham Lincoln married Mary Todd.