Not it's not, only the Septim bloodline starting with TIber Septim (Talos), also nicknamed Dragon of the North or Ysmir. Reman and Alessia were not Dragonborn.
St. Alessia and the line of Reman Cyrodiil both possessed the Dragon's Blood, remember it was Al-Esh who received the Amulet of Kings, sealing the covenant that allowed Akatosh to bestow Dragon's Blood upon the leaders of the Empire.. It is no specific 'line,' due to the many assassinations and dilutions of the Septim line, nobody in Tamriel could claim Uriel Septim VII as a direct descendant of Tiber Septim and keep a straight face . Martin Septim was Dragonborn as well, but it is more a divine connection to Akatosh than a bloodline.
i think he was, i read about him, he WAS born in atmora, the nord's original homeland, under the name of "talos"...
Some sources say he came from Atmora, some say he was born in High Rock and trained in Skyrim. Considering his godhood and CHIMliness both possibilities might be true, divinity makes everything more complicated.