I'm pretty sure that it's established that Natalie is Andre's blood sister. But what I am sure about is that in Ep #368/369, Natalie informs Joshua that she was born with her title.
Ahh, the arrogant french.
She is probably so snooty about these backwater neo-Americans that figures she could say anything to Joshua and he'd believe it...or per parents actually
named her her "Countess"!
Can lower aristocratic titles be inherited or is it possible that a titled relative can bequeath his or her estate including the title in his/her will?