Viola Davis has spoken about seeing the effects of sexual assault first hand.

The Help actress says her sister was sexually assaulted at age eight and is now a heroin addict and prostitute.

Viola also described her mother and herself as survivors of sexual assault on Sunday at a benefit for the Rape Foundation.

It provides free medical treatment, counselling and legal aid to rape victims plus special services for children.

Viola has been an advocate for the organisation since playing its founder in a 2010 film.

The actress was among the guests of honour at the organisation’s annual fundraising brunch.

The event was held at billionaire Ron Burkle’s Greenacres estate in Beverly Hills, California.