There is another theory that Nobushige SANADA was actually the oldest son and Nobuyuki was the second son, but they were switched because Nobushige's mother was of lower birth.
真田信繁の方が実は長男で信之が実は次男なのだが、信繁の母の方が身分が低いので長男と次男を入れ換えられたという異説がある。