There was another member of the family, his cousin Jacques-François de Borda, who would have a major influence on Charles as he grew up. Jacques-François was also born in Dax and was fifteen years old when Charles was born. Jacques-François had a passionate love of mathematics and science, and was in contact with the leading mathematicians of his day.
Nagkaroon ng isa pang miyembro ng pamilya, ang kanyang pinsan Jacques-François de Borda, na magkaroon ng isang malaking impluwensiya sa Charles bilang siya lumago up. Jacques-François ay ipinanganak din sa Dax at noon ay labing-limang taon gulang na kapag Charles ay ipinanganak. Jacques-François nagkaroon ng isang makabagbag-puso pag-ibig ng matematika at agham, at sa makipag-ugnayan sa mga nangungunang mathematicians ng kanyang araw.