Prince's alleged niece and grandniece are officially in the game ... filing documents they believe prove they ARE blood relation to his royal purpleness.
Turns out Prince's deceased half brother ... Duane Nelson Sr. had a daughter, Brianna ... and a granddaughter, Victoria. Both filed documents in Prince's probate claiming they are rightful heirs.
For evidence, they included Duane Sr.'s 1958 birth certificate ... which lists John Nelson as Duane's father -- same as Prince.
As we first reported ... an attorney alerted the court of Victoria's existence. Brianna, on the other hand, came outta left field.
Family sources tell us Tyka Nelson and the other established siblings have questioned whether Duane was actually blood. The new docs are a good start -- showing Duane's direct link to Prince -- but Victoria and Brianna will most likely have to submit to DNA testing as well ... to get a cut of the inheritance.