Answer by taking a quote from the following article:

Marion Eleanor Zimmer Bradley (June 3, 1930 - September 25, 1999) was an American author of fantasy, historical fantasy, science fiction, and science fantasy novels, and is best known for the Arthurian fiction novel The Mists of Avalon, and the Darkover series. While some critics have noted a feminist perspective in her writing, her popularity has been posthumously marred by multiple accusations against her of child sexual abuse and rape by two of her children, Mark and Moira Greyland, among many others. Zimmer Bradley's first child, David R. Bradley, and her brother, Paul Edwin Zimmer, also became published science fiction and fantasy authors.

In 2014, her daughter, Moira Greyland, accused her of sexual abuse from the age of three to 12. In an email to The Guardian, Greyland said that she had not spoken out before because:  I thought that my mother's fans would be angry with me for saying anything against someone who had championed women's rights and made so many of them feel differently about themselves and their lives. I didn't want to hurt anyone she had helped, so I just kept my mouth shut.  Greyland also claimed that she was not the only victim and that she was one of the people who reported her father, Walter H. Breen, for child molestation, for which he received multiple convictions. By her own admission, Bradley was aware of her husband's behavior although she chose not to report him.  In response to these allegations, on July 2, 2014, Victor Gollancz Ltd, the publisher of Bradley's digital backlist, announced that it will donate all income from the sales of Bradley's e-books to the charity Save the Children. The author Janni Lee Simner, who has continued to write works in Bradley's Darkover series, announced on June 13, 2014 that she would be donating advances from her two Darkover books, her Darkover royalties and at the request of her husband, Larry Hammer, payment for his sale to Bradley's magazine, to the American anti-sexual assault organization Rape, Abuse & Incest National Network.  Since the allegations were made public, Greyland and her brother Mark have spoken at length about their experiences  and a number of famous science fiction authors have publicly condemned Bradley. Among the first was John Scalzi, who within a day of the allegations being made public, described the allegations as "horrific". Hugo Award winner Jim C. Hines wrote "All of which makes the revelations about Marion Zimmer Bradley protecting a known child rapist and molesting her own daughter and others even more tragic." G Willow Wilson, World Fantasy award winner, said she was "speechless". Diana L. Paxson, who collaborated with Bradley on a number of novels and who continued to write novels set in the Avalon Series after Bradley's death, said that she was "shocked and appalled to read Moira Greyland's posts about her mother."

what was she accused of?
In 2014, her daughter, Moira Greyland, accused her of sexual abuse