Prince: Potential New Heir Out There ... If She Steps Up

Prince's fortune is about to be spread even thinner because he has a long-lost relative -- according to a lawyer for Prince's deceased half brother.

The attorney filed documents, obtained by TMZ, with Prince's probate ... saying his former client, Duane Nelson, Sr. has a granddaughter somewhere in Minnesota.

Duane died in 2011, and his son -- who died in 2005 -- had a daughter who is now 11 years old.

The attorney, Mike Padden, says the court should vet the girl, and wants a guardian appointed to look out for her interests in the estate.

Duane -- who was Prince's bodyguard for a while -- was omitted from the list of siblings when Tyka Nelson began probate proceedings. We're told some family members don't buy he was a blood relative.

That was never proven, but you can bet DNA testing would be part of vetting the grandniece.