Sharon Tate's Sister Tries to Block Manson Follower Leslie Van Houten's Parole

Sharon Tate's flesh and blood remains adamantly opposed to the proposed release of one of the Manson Family members ... the Family that slaughtered her sister and unborn child.

Debra Tate is lobbying California Governor Gavin Newsom hard ... asking him to reject the California Parole Board's recommendation to release Leslie Van Houten.

Debra wrote a letter to the Guv -- obtained by TMZ -- laying out all her reasons why the Manson Family murderer should remain behind bars for the rest of her life.

70-year-old Van Houten murdered Rosemary and Leno LaBianca back in 1969, the day after the slaughter at Sharon's house ... and Leslie's just been deemed suitable for release.

Debra doubts Leslie's been rehabilitated and says ... "the danger of letting these people out of prison is ever-present and real. I am scared, not just for myself and other family members who may be targeted, but for society."

Debra's not buying the argument Manson Family members don't pose a risk to society now that they're septuagenarians, saying, "Age is immaterial to psychopathology. There is no cure for the sociopath. The disposition to committing violent crime does not have an expiration date. And criminals do not retire."

For Leslie, it's the fourth time the board has recommended paroling her ... the previous three attempts have been rejected by the State's top leader.