After Kiu Fung received false information from Madame Ma that Deun Jing Tsun was the mysterious "Leader Big Brother" figure who had caused him so much trouble, Kiu Fung was bent on settling his vendetta against the man whom he thought was his mortal enemy.
Kiu Fung knew that Deun Jing Tsun was a prince of Dali, first in line of succession to the throne. He must have also known that Deun Yu, the new sworn brother he had met a few months ago, was the son of this Deun Jing Tsun (or at least a close relative).
Did the thought ever cross Kiu Fung's mind that if Deun Jing Tsun were indeed Leader Big Brother, that would put Kiu Fung at odds with Deun Yu, who was one of his last friends (other than Ah Chu)? Not that this would have stopped Kiu Fung, but it seems like Kiu Fung never even thought about it.
The matter was also tactfully not discussed between Kiu Fung and Deun Yu when they met again later in DGSD.