Sian Barbara Allen has died aged 78

Her sibling said in a Facebook post confirming her death: "My wonderful sister, actress Sian Barbara Allen died peacefully today after a long illness. This loss is too hard."

Sian died from Alzheimer's disease on Monday in Chapel Hill, North Carolina, according to an online obituary.

Born in Reading, Pennsylvania, on 12 July 1946, she studied acting at the Pasadena Playhouse from 1964. She made her television debut in 1971 in 'Alias Smith and Jones' and became a familiar face on television.

Her performance as a mentally challenged young woman in the 1972 film 'You'll Like My Mother' earned her a Golden Globe nomination in 1973 for Most Promising New Actress.

While working on the film, she began a relationship with her co-star, Richard Thomas, and they were together for several years, according to IMDb.

Sian appeared in two episodes of 'The Waltons' in 1973 as Jenny Pendleton, portraying a love interest for Thomas's character, John-Boy.

That same year, she starred alongside Bette Davis in the television thriller 'Scream, Peggy, Scream', portraying a young woman hired to care for Davis's character, unaware that the previous caretaker had been murdered.

She took on a leading role opposite Gregory Peck and Jack Warden in the 1974 film 'Billy Two Hats' and in 1976, she portrayed Anne Morrow Lindbergh in the fact-based television movie 'The Lindbergh Kidnapping Case'.

Throughout her career, she made multiple appearances on 'Marcus Welby, MD', each time as a different character, and guest-starred on 'The Rockford Files' with James Garner, 'Hawaii Five-0' with Jack Lord and James MacArthur, and 'Ironside' with Raymond Burr.

Sian later explored writing, penning an episode of 'Baretta' in 1978, in which she also starred as Baretta's cousin, Ellen.

Her final acting role was in 1990 on 'L.A. Law' before she stepped away from the industry to focus on her family.

In 1979, she married fellow actor Peter Gelblum, and they had a daughter, Emily, named after the character Sian played in a 1976 stage production of 'Our Town'.

The couple divorced in 2001.

Sian is survived by her daughter, Gelblum, her sisters, Hannah Davie and flash fiction author Meg Pokrass, her grandson, Arlo Fonseca, as well as her nephew Miles Bond.