Earl Spencer has said he welcomed the findings of the Dyson report into the BBC but believes there are still "very big questions" that remain unanswered.
The inquiry concluded this summer that the BBC fell short of "high standards of integrity and transparency" over Martin Bashir's 1995 interview with his late sister, Diana, Princess of Wales.
Her brother told BBC Breakfast some at the corporation behaved in a "truly abysmal" and "possibly criminal" way.
The BBC stressed it had apologised.
Speaking to BBC Breakfast's Sally Nugent on Monday, Earl Spencer said: "Actually, Lord Dyson did a very good job. His brief was tiny - it was to look at a very specific area. And there's still so much more to look at in the broader terms of who was responsible for what?
"How how did it come to this? And did documents get hidden from view? And all sorts of really important stuff which has yet to come out.
"So I see the Lord Dyson report as a very welcome development but there's still a long way to go with this."
He added: "There are very big questions still out there for me, as to the broader picture of how something like this could have happened.
"It's clear to me that there there are certain people who were in the BBC, who have behaved in a way that is truly abysmal and possibly criminal."
The Dyson report concluded in May that Bashir acted in a "deceitful" way and faked documents to obtain a 1995 Panorama interview with Diana.
It said the BBC's own internal probe in 1996 into what happened was "woefully ineffective".
In response to Earl Spencer's comments on Monday, a BBC spokeswoman said: "Today's BBC has aimed to be as open and transparent as possible about the events of 25 years ago.
"We held an independent, judge-led investigation which was concluded within six months and, separately, BBC News commissioned an edition of Panorama in which journalists robustly investigated their own employer.
"While the BBC cannot turn back the clock after a quarter of a century, we made a full and unconditional apology the day Lord Dyson's report was published."
Bashir has also apologised, although Earl Spencer noted on Monday how the latter had "not apologised to me".
BBC News has contacted Bashir for a statement.
When asked how far he might be able to take any subsequent action, Earl Spencer - who did not rule out personal legal proceedings - replied: "That's the question. And I've got people looking at that and we'll see but it's not going to end now.
"I'm not saying that as some ugly threat. It just can't stop here because there's still more to come out."
"I'm quite determined," he went on. "I'm really sure that there's some very wrong things here."
In September, detectives said they would take no further action over the interview. The Metropolitan Police looked at an independent review of the methods used to obtain the interview, but had "not identified evidence of activity that constituted a criminal offence".
Elsewhere in the same interview at his family estate in Althorp in Northamptonshire, Earl Spencer revealed he had rejected The Crown's request to film at the property.
The forthcoming fifth series of the royal Netflix drama stars Elizabeth Debicki as Princess Diana and will reportedly dramatise her headline-making Bashir interview, as well as other key events in her later life.
"They applied," he said. "They wanted to shoot here. But I don't really do that stuff.
"Actually, to be honest, I don't watch The Crown so I just said, 'Thank you but no thank you'."
Diana died in a car crash in Paris in 1997.