Princess Usui's mother Otomi was originally a wife of Tadamasa MIZUNO and the biological mother of Ieyasu's mother Odai no kata, and therefore Tadatsugu was the husband of a sister of both of Ieyasu's parents and hence Ieyasu's uncle-in-law.
碓井姫の母・於富は元々、水野忠政の夫人で家康の母・於大の方の実母であったから、忠次は家康にとっては父母双方の妹の夫、義理の叔父ということになる。