Alan Bennett, brother of Brady victim Keith, has posted on the website dedicated to the search for Keith Moors grave.
On searchingforkeith.com Alan said:
It is good news that the police have finally been able to search Brady's room and also to make an arrest.
I would say that, until some definite information is found regarding Keith's whereabouts, we do not want to raise our hopes too high at this stage. Nonetheless, it is a very important development.
The lawyer for Winnie Johnson, John Ainley, has said that his client's family is treating recent developments "with some suspicion".
He told ITV News it was "very timely" that the documentary which brought the letter claim to light is due to be aired tomorrow.
Greater Manchester Police are investigating whether Moors murderer Ian Brady has written a letter to be opened on his death revealing the location of the body of one of his victims.
Moors murderer Ian Brady's legal advocate has been arrested for allegedly failing to reveal information about where one of his victims is buried, sources have confirmed.
The child killer has always refused to disclose the whereabouts of Keith Bennett's body.
His refusal has caused constant heartache for Keith's mother, who has terminal cancer.
Alan West, the stepfather of ten-year-old Lesley Ann Downley - a girl who also died at the hands of Ian Brady and Myra Hindley, says he's sceptical about today's developments.
A solicitor who used to represent Winnie Johnson, Keith Bennett's mother, has described the latest development as "extremely interesting and exciting".
David Kirwan, a managing partner with Kirwans law firm, said he had a series of face-to-face meetings with Ian Brady at Ashworth mental hospital in 2007. He said:
There is potential credibility to the claim that Brady may have written such a letter. I believe it is a human instinct not to take secrets to our grave.
I believe Ian Brady knows the exact whereabouts of Keith Bennett's grave and iscapable of directing police to it.
However, it is important to remember who we are dealing with and how he seems to enjoy bizarre mind games and manipulation.
During interviews Brady told me mischievously how when he was on the moors with the police in 1986 he walked them across the graves of his victims.
This photograph shows Jackie Powell, Ian Brady's mental health advocate, who has claimed that Brady gave her a sealed envelope addressed to the mother of one of his victims.
The revelation raised hopes that the envelope may contain the location of Keith Bennett, the only one of Brady's victims whose body has not been found.
Ms Powell says she no longer has the envelope and that her words have been "misrepresented". Greater Manchester Police are investigating.
The lawyer for Winnie Johnson, John Ainley, has said he is not "holding his breath" that recent developments will lead to the location of her son's body.
He told Sky News: "It may be at this late stage that he [Ian Brady] has got some sort of conscience about all of this" but he said that he would be surprised.
Mr Ainley added that Ms Johnson's family is "reassured that the police are investigating" but that he did not think its was advisable to raise her expectations.
No new date has been set for Ian Brady's mental health tribunal.
It is understood that arrangements need to be put in place to agree on a suitable date for all legal parties in the case.
Brady is challenging his insanity diagnosis which allows doctors at Ashworth mental hospital in Merseyside to force feed him.
It is believed he wants to return to the normal prison system where staff would not be allowed to stop him going on hunger strike.
For more on this story, visit the ITV national news site.
Greater Manchester Police have said there are no immediate plans to search Saddleworth Moor, near Manchester, where he and Myra Hindley buried four of their five victims.