Brittney McInnes's stepfather, Bradley Rietze, pleaded guilty against the advice of his lawyers on Monday to killing the 17-year-old. Brittney McInnes's stepfather, Bradley Rietze, pleaded guilty against the advice of his lawyers on Monday to killing the 17-year-old. (Facebook)

The mother of 17-year-old Brittney McInnes told a packed Calgary courtroom during victim impact statements Wednesday she feels guilty for allowing the teen's stepfather and murderer, Bradley Rietze, into their lives.

"I have no idea how I made it through the first year," Kelly Wallace said. "I cry for her many times in the day.... I am so angry."

She said her family is broken and will never be whole again.

"I can't stop this pain in my chest, I have a real broken heart," she said. "This person who I trusted, who Brittney trusted, should have been there to protect us."

Rietze, 47, surprisingly admitted Monday he strangled the teen while raping her, just as his first-degree murder trial was about to start.

Stepfather apologizes

Rietze has never come forward with a reason for raping and killing his stepdaughter, but did apologize for it publicly in court on Wednesday.

"My one wish is to bring Brittney back," he said. "I promised I would always be there to protect you.... Sorry will never be enough."

Katie McInnis, Brittney's sister, said every year on her birthday it's just a reminder that it was the last time she saw her sister alive.

"Losing her was the worst thing anyone could put me through," she said.

Rietze's sister JoAnne Peterson said she was proud of her youngest brother for changing his plea so the "nightmare" of what happened wouldn't be played out in front of the court.

"It's so hard to see one of your own depicted as a monster," she said.

Reitze sentenced to life in prison

First-degree murder in Canada carries with it an automatic sentence of life with no possibility of parole for 25 years.

McInnes’s body was found in her Canyon Meadows home in January 2010 after her mother had reported the 17-year-old missing.

When police searched the home, they did not find the Grade 12 student. It was hours later that a family member searched the house and found her body under her bed.

The court heard Wednesday Rietze made McInnes's bed after he stuffed her body under it, placing her teddy bear in its usual spot.

Rietze, who was the girl's stepfather for 13 years, was charged with the her slaying a few days after her body was found.

Rietze pleaded guilty against the advice of his lawyers.