Anne of Brittany’s father, Francis II of Brittany, did have some other children besides Anne of Brittany and her sister Isabeau of Brittany.
He had one legitimate son with his his first wife Margaret of Brittanny, John (29 June- 25 August 1463), Count of Montfort, whom unfortunately died within months.
He also had 4 illegitimate children, 2 sons and 2 daughters, with his chief mistress Antoinette de Maignelais which had before been the chief mistress of Charles VII untill his dead in 1458. She had 2 illegitimate daughters with him as well. She died at Francis II in 1474.
Francis II still had a big problem though because even though he had 4 children he had no heirs that could reign after he died.
So the same year his chief misstress died 1474 he married Margaret of Foix. Margaret of Foix gave him 2 legitimate daughters but no sons. The oldest, Anne of Brittany, was born in 1477 and her sister Isabeau was born in 1478.
Unfortunately for Francis II according to semi-Salic law women were not allowed to reign after their father passed away unless there was no male relative at all to take the throne. If there was son, an uncle or cousin or nephew the daughters would be ignored and the throne would go to the male relative!
Francis II would have to either produce a male heir before his death or get his daughter Anne of Brittany recognized as his heir.