Donny Osmond speaks to ET about what caused his sister Marie to become so ill that the duo had to cancel their Las Vegas show on Friday night.

Donny reveals that Marie got sick with iodine poisoning after she stopped at a sushi restaurant on the way to the show and ate something containing peanuts and shrimp -- both of which she is highly allergic to.

Donny tells ET he rushed to be with his sister when he learned Marie was ill. When he arrived, a doctor was already on the scene and Marie was lying on the floor surrounded by people. He immediately canceled the show, ushered people out of Marie's dressing room and held her hand until she went to sleep.

Marie recovered well and was able to perform during last night's show at the Flamingo Las Vegas, said Donny, who added that it was the first time he had to cancel a show.