Mueller reportedly asked Richards to help care for her three-year-old boys Max and Bob before she checked into a treatment facility to deal with her prescription pill problems.
And the former Bond girl was happy to oblige, reported E! online.
"Richards will look after the tots for an indefinite period of time as Sheen is busy working on his TV sitcom 'Anger Management',"a source said.
Richards, 41, has been supporting Mueller through her drug battles and multiple rehab stints in recent years, and previously took care of Sheen's boys while their mother stayed at a wellness centre to aid her recovery earlier this year.
Richards has two children of her own with Sheen, daughters Lola and Sam, and she adopted baby girl Eloise last year.
The actors split in 2006 and Sheen went on to marry Mueller in 2008. They divorced in 2011.
