"This is not normal behavior for her." a source tells ET of Mueller, who has a history of drug abuse
. "She has relapsed," the source believes.
On Wednesday, Salt Lake City Police confirmed to ET that Mueller and her 7-year-old twins with Sheen -- Max and Bob -- had been found safe after they received a welfare check call. Mueller was taken to an area hospital for evaluation. A public information officer for the Utah Highway Patrol told ET on Thursday that the incident involving Mueller and her children is "still under investigation."
Mueller's sister, Sydney Wolofsky, tells ET that Mueller currently remains hospitalized but does not know if she has relapsed. Wolofsky says she and Mueller last spoke two days ago, and that her sister seemed "fine" at the time.
"That's what is so weird," Wolofsky says. "There are usually signs leading to a relapse with addicts and this time there weren't. We spoke about how the boys are doing in school, a fundraiser they just did... It was sister talk. It was normal."
For now, Mueller's parents are by her side.
"My mom is with Brooke now in the hospital," Wolofsky says. "My dad is with the boys [Bob and Max], they are totally fine. When I asked about how Brooke is doing, my mom said Brooke is going to be totally fine. It's always scary when you get these calls. It makes me feel better knowing that my mom is with her."
"We have been through this before," she adds. "I'm not scared because Brooke is in good hands.”
Meanwhile, 51-year-old Sheen addressed Mueller's current situation with a bizarre poem on Thursday. The two wed in 2008, but filed for divorce in 2010.
"In the face of a roughshod and polar moment of unsanctioned divide, sanity and certainty were restored with grace and resolve," Sheen told People
. "Immeasurable gratitude and love to a fellow yeoman of the apocalypse; sir Michael from that hamlet called Walters, need accept each and every Crimson palm-sting, as high fives rain upon his bitchen nobility, not just from this traveler, but from his safe and stable brood as well …"
The incident on Wednesday isn't Mueller's first run-in with the law. She was arrested for cocaine possession
in December 2011, after allegedly assaulting a woman in an Aspen, Colorado, bar. She was sentenced to 12 months probation and entered a rehab program. In May, Mueller checked herself back into rehab.
For more on Sheen and Mueller's relationship, watch the video below.
-- Additional reporting by Brendon Geoffrion