Rae Sremmurd's brother is facing a first-degree murder charge -- this after cops say he shot his own father, RS's stepdad, in cold blood.
Swae Lee and Slim Jxmmi's half-bro, Michael Sullivan, has been charged with murder in Lee County, MS ... according to D.A. John Weddle, who tells us the indictment was handed up after a grand jury heard evidence in January.
We're told this is not a death penalty case -- but Michael still faces life in prison if convicted. He's still in custody ... no word when he's due again in court, or when the case will go to trial.
The D.A. wouldn't get into details of the alleged killing, other than to say Michael's biological father, Floyd Sullivan Jr., was shot at least once on Jan. 6, 2020 -- which is when word of his death first surfaced and Michael was detained as a person of interest.
As we told you ... Michael's mother, Bernadette Walker, told us she believed Michael might have been off his medication before the alleged shooting -- as she explained he'd been having episodes with her too before going to live with his dad.
Bernadette also revealed Michael is afflicted with Schizoaffective disorder, which lends to schizophrenia and everything that comes with it ... including mood swings, delusions, etc. It's unclear how this factored into what was presented to the grand jury -- but we were told he was held for a psychiatric evaluation before he was eventually arrested and taken to jail.
As for Swae and Slim ... they haven't said too much about the matter, but the former did tweet broken heart emojis shortly after the incident, as well as this in a since-deleted message ... "I pray this world let's me keep some compassion."
The 2 rappers appeared somewhat close with Michael, as they'd previously taken pics together and seemed pretty happy.