PARIS – Princess Antoinette of Monaco, the late Prince Rainier III’s older sister and a prominent advocate for animal rights, has died at age 90.

The Prince’s Palace said Friday that she died overnight at Monaco’s Princess Grace Hospital.

The palace did not provide a cause of death.

Princess Antoinette was the daughter of Princess Charlotte and Prince Pierre.

She was involved in the cause of animal rights and was the president of Monaco’s Society for the Protection of Animals.

Prince Albert II, the son of Rainier and U.S. actress Grace Kelly, ordered all the flags in the principality to be at half-staff.