Michael Clarke Duncan's Family -- We Want Omarosa Investigated

Michael Clarke Duncan's family has questions about Omarosa Manigault ... specifically, whether the late actor's fiancee unduly influenced him into re-writing his will months before he died and leaving almost everything to her.

Judy Duncan, Michael's sister, tells TMZ she's hired a lawyer to investigate the circumstances surrounding the change to the actor's will in April 2012, making Omarosa the main beneficiary.

Judy believes MCD was not of sound mind when he made the changes ... because as early as December 2011, the actor was not himself, slurring words and stumbling around.

Judy says her suspicions about Omarosa intensified when MCD was hospitalized following his heart attack ... telling us Omarosa was fixated on MCD's money when he was on life support.

Another thorn in Judy's side ... Omarosa has already sold a bunch of MCD's personal effects (watches, cars, his "Green Mile" director's chair, awards, etc.). Judy says Omarosa sold the stuff without the family's knowledge ... and she's pissed.

It's worth noting: MCD reportedly left Judy $100,000 in his will ... so it's not like she got squat.

For Omarosa's part -- she tells TMZ, "I don't control the estate or the finances and Judy knows it. If you saw all of her emails and texts to me you would see that she is just trying to get money from me, and threatened going to press if I did not give it to her and that is a crime!"