Why did the old man wait 3 years before telling Bruce Lee about what happened to his sister?
Also, before the old man finally revealed the truth -- a secret he had been sitting on for 3 years -- didn't Bruce ever wonder what made his sister stab herself with a shard of glass?? Wasn't he the least bit curious about it? Didn't he ever talk to the cops about it, or do any investigating on his own?
My guess is he did know what happened, the reveal from the old man was that it was Hans men that were responsible.
The old man probably held back this info out of fear that Lee would go for revenge, or get himself killed if he knew the truth. They probably have some peaceful beliefs that would go against revenge. But with Lee going anyway, the old man decided to reveal what he knew.
The real reason though is that the story needed to give Lee motivation to go. Otherwise why would this character risk his life?