Veteran actress Patty Duke is mourning the death of her older sister following a mystery illness.

The Miracle Worker star, 66, broke the sad news to fans on her blog on Monday (06May13), although few details were known about her sister Carol's passing as Wenn went to press.

She simply wrote, "Thank you all for the love and prayers for the passing of my sister, Carol."

Duke's sibling was recently hospitalised and Duke regularly reached out to her devotees as she struggled through the tough time.