Johnny Lewis -- Baby Mama Claims Johnny's Dad is Screwing Baby Over

Johnny Lewis -- who died in a murderous rampage in 2012 -- left behind a young daughter who's being cheated out of tens of thousands of dollars by Lewis' father ... so says the baby mama who's battling the family in court.

Lewis -- who starred in "Sons of Anarchy" -- died in Sept. 2012 after killing his 81-year old landlady, dismembering her cat and then falling to his death from the roof of her home.

Diane Marshall-Green filed legal docs -- obtained by TMZ -- seeking to remove Johnny's dad, Michael, as administrator of the Estate. She claims Michael refuses to recognize 3-year old Culla Mae as Johnny's sole heir ... even though Johnny signed a declaration of paternity.

Court docs show Lewis was worth about $41K when he died.

Marshall-Green claims Michael is denying Culla her rightful inheritance ... and has refused to fork over $37K Johnny owed in back child support.

Sources close to Marshall-Green tell us ... if Culla Mae gets the money ... it will go into a trust account.