The Artois Family were Junior Branch of the Frankish royal family, know as the Capets.
The Capets were named after the first member of their dynasty. Hugh Capet. Hugh was “elected ” because of his greater nobility— his messianic blood lines.
Blanche d’Artois married both Edmund Crookback of England and Henry of Navarre. She had children —Juana Of Navarre , Thomas of Lancaster and Henry of Monmouth.
The Idea of carrying a messianic blood line went to Thomas’ head. he rebelled against Edward II and was eventually beheaded for his troubles.
Edward II married Isabella of France. It does not really matter who was Edward III’s father.
Isabella passed the same messianic line onto him.
What the house of Lancaster pursued was not Edward’s lack of possession of the messianic line, it was that if Roger Mortimer was his father then Edward III and his descendants had no right to the English throne.