A Lethbridge woman says she believes the man charged with killing her sister, brother and sister's boyfriend was motivated by jealousy.

Kyle Devine, 27, Clarissa English, 24, and her brother Dakota English, 18, were found stabbed to dead in the Lethbridge townhouse they shared. It happened early in the morning on April 28.

A 21-year-old man was charged on Thursday with three counts of second-degree murder in connection with the slayings.

Kyle Devine

Kyle Devine, 27, was described on Facebook as "in a relationship" with Clarissa English. (Facebook)

Chantelle English says the suspect wanted to date her sister.

"They hung out a few times and I know he liked her, he wanted to be with her, but she didn't want to be with him," she said.

"And she started dating Kyle shortly after her and [the suspect] were hanging out."

The suspect had been charged with assault with a weapon and uttering threats on April 9.

The alleged victim in that incident, Kyle Devine, was one of the three people killed two weeks later.

Investigators believe the accused was with the victims in the hours prior to their deaths, in breach of the conditions of his release from custody in connection with the assault charges.

He was arrested on April 29 and was being held at the Lethbridge Correctional Centre when he was charged this week with the homicides.

The suspect's next court appearance is set for May 21.