Diddy and longtime girlfriend Cassie are together again, and Cassie has committed to helping him raise his twin daughters.

Yesterday, Diddy's longtime friend and baby's mother Kim Porter passed away. She was raising Diddy's twin daughters D'Lila and Jessie. Now that she's gone, the twins will be moving in with their father, MTO News has learned.

And Diddy will have help raising the twin 10-year-old girls. MTO News confirmed that yesterday Cassie and Diddy reunited. Our insider explained, "Cassie told [Diddy] that she's there for him. And she's moving back into [Diddy's $40 million Bel Air mansion]".

The insider claims that Diddy is absolutely devastated by the news of Kim's passing, but he's holding himself together for his kids. Our snitch added, "Diddy is all about his children now. That's his main focus. And Cassie is his ride-or-die, so she's there for him."

We're told that Cassie has committed to helping her longtime love raise his two youngest daughters.

According to reports, sources close to Diddy say he never stopped interacting with Kim, despite breaking up way back in 2007. We're told their connection went beyond the fact they were co-parenting 3 kids ... they genuinely had love and affection for each other.

They celebrated holidays as a family, vacationed together and just hung out ... and there was never an extended period where they were at odds with each other.

Porter was found dead at her Toluca Lake home Thursday, and the emergency dispatch revealed a call came in for a patient in cardiac arrest. A source told us Porter was recently sick with pneumonia.

She leaves behind their 20-year-old son, Christian, and twin, 10-year-old daughters Jessie James and D'Lila ... as well as 27-year-old Quincy whose dad is Al B. Sure!.