In 1579, Tokugawa Ieyasu did the unthinkable. He ordered his wife Tsukiyama-dono (she was known to selfish and wicked) and son Nobuyasu to death. In modern times, nobody in their right mind would kill their wife and son. Ieyasu had to made a decision, it was his family or the Tokugawa house. The last thing Ieyasu wanted was a fragmented Tokugawa house, so he had to do the unthinkable. He had no other choice since Nobunaga carried the whip in the Oda/Tokugawa Alliance. Elisonas and Lamers did a great job explaining and it is one of the best so far.
The Chronicle of Lord Nobunaga (p. 38)
"In 1579 Tokugawa Ieyasu's son and heir Nobuyasu and his mother Tsukiyama Gozen, Ieyasu's wife were denounced to Nobunaga for atrocious conduct and treasonous activities. Nobunaga demanded that Ieyasu put them to death; Ieyasu complied, forcing Nobuyasu to commit hari-kiri and having Lady Tsukiyama executed. According to a frequently repeated story, none other than Nobuyasu's wife, Nobunaga's daughter Gotoku, wrote her father the letter that incriminated her husband and mother-in-law. The author of this story, Okubo Hikozaemon reported that on hearing Nobunaga's verdict condemning his son, Ieyasu reacted with the words: 'It is something that cannot be helped. I bear Nobunaga no rancor.... As long as I am locked in conflict with a great enemy [Takeda Katsuyori] and depend on Nobunaga to back me up, I cannot very well defy Nobunaga. It cannot be helped.' In other words, Ieyasu had concerns that transcended his parental instincts; the survival of the house of Tokugawa was at stake.
Ieyasu is universally described as Nobunaga's ally. Yet the willingness to accept an intolerable demand without protest is a characteristic not of the ally but of the subordinate. If a special relationship existed between these two, it was skewed in favor of Nobunaga, who retained the whip hand."
Nobunaga no tame!