The loved-up couple will have to wait until Brett is legally divorced before they can make the trip down the aisle, making it Charlie's fourth time.
Brett, 24, is in the process of divorcing her husband, according to RadarOnline, and is being represented by Charlie's lawyer Mark Gross. The proceedings are expected to be resolved soon.
Jonathan, who will be representing himself in the case, filed for divorce last July. His estranged wife didn't file a response until January this year, two weeks before Brett announced her engagement. Jonathan and the adult film star had been married for about a year and do not have children together.
"The divorce between Brett and Jonathan is going to be finalised in the next few months, if not sooner," an insider told the website. "There isn't anything that is being contested."
Charlie's representative also confirmed that the Anger Management star knew about his fiancée's situation and didn't have a problem with it.
"She's been separated from this guy for two years," the spokesman told People. "It's just a matter of the paperwork winding its way through the court system."
"Charlie and Scottie have no secrets," he added, referring to Brett's nickname. "Charlie is well aware of Scottie's situation."
The Hollywood actor, 48, popped the question while the couple were enjoying a romantic Valentine's Day getaway in Hawaii. Charlie took Brett by surprise when he proposed shortly after the lovebirds had watched the sun rise.
The blonde beauty gushed over her fiancé the day before he proposed, tweeting, "Happy Valentine's day to the love of my life & my best friend! Everyday is Vday when I'm with you @charliesheen."
The couple first went public with their romance last November when Charlie and Brett were pictured canoodling on the balcony of their hotel in Cabo San Lucas, Mexico.
Earlier this year, Charlie sent the rumour mill into overdrive when he pretended that he and Brett had tied the knot during a holiday in Iceland.