Selina Siwy spoke to CBC after police found the 27-year-old mother and her children dead in a mobile home in Tisdale, Sask., on Wednesday morning.
The body of the man believed to have killed them was found in a home in Prince Albert five hours later. A six-month-old girl, who was not hurt, was also found with the man.
Police did not name the man, but CBC News has identified him as Steve O'Shaughnessy, also known as Steve Manley.
Siwy told CBC in a brief interview that O'Shaughnessy had been in a relationship with Gosling for about three years. They had been living together in Tisdale. In recent years, he had lived or worked in Nipawin and St. Walburg.
The issue of O'Shaughnessey's mental health was discussed by Siwy in an online forum.
"You all don't know the whole story behind the situation," she said. "We have been trying to get him help for years and we thought he was in the clear."
According to posts O'Shaughnessy made on social media, he had worked in the oil patch and had a baby girl with Gosling. It's believed to be the same baby found with him at the home in Prince Albert.
Siwy said in an online forum that her family loved her brother and Gosling and her children, and are in a state of shock now.
"We loved him then and we love him now," she said in a post on the Square-pic.com website. "We loved Latasha and the kids as our own as well."