[Atlarax]

Speculation: Depending on the age of Hayate's "brother", Hayate and Nagi may be related.

Hayate's true parents may be the result of the relationship of his "brother" and Nagi's mother, explaining his resemblance to Nagi's mother.

Afterwards, to save her love from being sold to the Yakuzi by mom and dad, Nagi's mom marries into the Sanzenin family for money (offered by the grandfather) and gives birth to Nagi, while Hayate is raised to believe his true parents are his mom and dad.

The money thing is hinted by Nagi's words to his grandfather "some people don't understand no matter how much money you use she won't come back."

Also, it seems at age 8 was the last time Hayate sees his brother, and it was also the time Nagi's mother died.

edit: o shi-disregard that. seems Nagi's mother didn't even know Hayate's brother. 187 shows her saying "thank you, stranger"

EDIT EDIT: Unless the elder Sanzenin specifically instructed Nagi's mother to forget about Hayate's brother, to suit his own son's needs.