In 1556, Nobunaga's father-in-law, Saito Dosan (1494-1556) was killed at the Battle of Nagaragawa. There was bad blood between him and his son Yoshitatsu. The bad blood started in 1553 when Nobunaga and Dosan met at Shotokuji. Once Dosan knew Nobunaga was not an idiot and a genius, the relationship between him and Yoshitatsu fell apart. Probably Yoshitatsu was jealous of Nobunaga. He most likely knew Nobunaga was not a fool, but too embarrassed to express his opinion.
The Chronicle of Lord Nobunaga (p. 103)
"Nagai Chuzaemon took on Dosan, laid hold of him as he was raising high his assault sword, and locked him in am embrace. Just when Nagai had almost succeeded in taking Yamashiro Dosan alive, Komaki Genta, a ferocious samurai, came running up, mowed down Dosan crosswise at the shins, and took his head as he fell prostrate."
Another reason why Yoshitatsu rebelled was that he was not the son of Dosan, but the son of Toki Yorinari. This was untrue. Toki Yorinari did have Miyoshino as a partner. When she became Dosan's concubine, she was not pregnant at that time. So in my opinion, Yoshitatsu made up this excuse to give him a reason to attack Dosan.
Nobunaga no tame!