Mrs Merrin said she was expecting a decision to be made in January but had since been told it would take weeks due to new legislation.
She said that no-one had warned her or her daughter that there might be delays, and accused parole staff of treating them like an inconvenience.
“It’s the fact that you’re not informed,” she said. “When you are, it’s all last minute. I can tell that we are just an utter inconvenience.
“The delay just feels like the last straw. I can’t be the only one who’s had this kind of problem.
“I appreciate that means he’s not going anywhere, but that’s not what this is about.”
She added: “This is about the process and the additional angst and strain it puts on families.”
The Parole Board and the Ministry of Justice have been approached for comment.
Sgt Forth, who had been a police officer for 14 years, was murdered after being called to a domestic disturbance after Weddle had smashed the window of a house where his ex-girlfriend was babysitting with her new boyfriend.
Having consumed alcohol and drugs including Temazepam, Weddle hit the officer with a piece of fencing before stabbing him multiple times.
In 2013, Weddle won a High Court case in which he claimed not enough was being done to aid his rehabilitation.