Kristen announced the news on Twitter, writing, “Yesterday my beautiful younger sister Julie lost her battle with addiction,” adding a broken heart emoji.
Johnston continued, “I know she's finally at peace. She had the best belly laugh in the world.”
The post included a sunny photo of the pair, in which Kristen has her arms around Julie.
The 52-year-old “Mom” star has opened up about her own addiction issues in the past, including in her 2012 memoir, “Guts: The Endless Follies and Tiny Triumphs of a Giant Disaster.”
At the time, she told Entertainment Weekly, "The most important thing I want to express to people is that I'm not cured. I could probably relapse in a minute. Who knows? It's just a weird disease that sneaks up on you and all of a sudden you're boozing at the bar, or whatever. And it doesn't have to be because of you or pressure or this-or-that. It just can be."
Johnston continued, "The most important thing is that I didn't want to set myself up for failure and be like, 'Look at me!' I wanted to write the book that I needed when I was suffering."