Tegwen Roderick from Abercanaid, near Merthyr Tydfil, received "serious but unexplained" injuries at Ysbyty Cwm Cynon in June 2014.
Several arrests were made after her death six months later, but the Crown Prosecution Service said they had "no further interest" in the case.
Mrs Roderick's family expressed concern about the care she had received.
The decision not to prosecute was made on 20 January, and Coroner Andrew Barkley told a pre-inquest hearing at Aberdare Coroner's Court it was "regrettable" he had not been informed sooner in order to move quicker to a full inquest.
He said there would be a fuller pre-inquest review in about a month, and a full inquest would follow as soon as possible.
During the police investigation three women and a man were arrested on suspicion of conspiring to pervert the course of justice and two of the women also faced assault allegations.
There will be no further action after a woman, 88, was injured at a Rhondda Cynon Taf hospital and later died.