NEW DELHI—It has all the makings of a bestselling novel. An Indian maharaja crowned as a toddler and rich beyond imagination falls into a deep depression in old age after losing his only son.

After his own death a few months later, his daughters, the princesses, don’t get the palaces, gold and vast lands they claim as their birthright. Instead, they are given a few dollars a month from palace officials they accuse of scheming to usurp the royal billions with a forged will. The fight rages for decades.

On Saturday, an Indian court brought this chapter to a close, ruling that the will of Maharaja Harinder Singh Brar of Faridkot was fabricated. His daughters will now inherit the estimated $4-billion estate, instead of a trust run by his former servants and palace officials....