The letters acknowledged the pain and suffering that their loved ones experienced in the care of the Belfast Health and Social Care Trust.
Glynn Brown's son Aaron is unable to communicate and has round the clock support.
The report from the Muckamore Abbey Hospital Inquiry has been delayed again this time until June.
A campaign group says the case came to light after CCTV material was reviewed, leading to 12 members of staff being fired and one prosecuted.
In a statement its Chair Tom Kark KC, said he had hoped to publish his findings towards the end of this year,
Dawn Jones' son Timothy had been in the Antrim-based facility for 12 years but is now in his own bungalow.
The Muckamore Abbey Hospital Inquiry panel is asking for information about recent and ongoing experiences.
BBC News NI hears from Glynn Brown, whose son Aaron was a patient at the County Antrim hospital and Dawn Jones who provided evidence about her son Timmy's abuse.
The final public hearing in the Muckamore inquiry is told that it is a "sincere hope" that families are given the answers they deserve.
