Sharon Tate's Sister Worries Manson Family's Leslie Van Houten Will Kill Again

Leslie Van Houten is likely to kill again after being released from prison -- so says Sharon Tate's sister ... who's deeply offended California cut loose the former Charles Manson follower.

Debra Tate tells TMZ that she's disappointed in the criminal justice system in the state, and especially the governor's office ... which she thinks really dropped the ball in not fighting harder against Van Houten's release.

While Gov. Gavin Newsom said he isn't happy with the decision handed down by the Court of Appeals -- siding with the parole board releasing Van Houten -- he also said he wasn't going to push back, arguing it was a losing effort.

Debra says Newsom could've taken it to a higher court, which is true, and she's taking his surrender as a huge slap in the face to the families of Manson's victims.

As far as how those loved ones feel about Van Houten being a free woman again ... well, she tells us they're absolutely terrified because there's a fear she could resort to her killing ways again in a heartbeat.

Clearly, Debra doesn't buy the parole board's belief Van Houten's rehabilitated. Her rationale boils down to the nature of the crime itself, and the mindset Van Houten was once under. She says once somebody goes down that dark path, in her eyes, it's easy to go back.

Now, while Van Houten wasn't involved in the Tate murders -- remember, it was the LaBiancas she helped butcher -- she's publicly said she really wanted to be present for the slayings of Sharon and 3 friends.

Debra finds that very troubling, and says that's why she has every right to weigh in here.

She adds ... the victims' families are outraged Van Houten gets to live out the rest of her life outside of prison -- despite a life sentence they assumed would be imposed forever more.

Debra's been leading a charge to keep all the Manson family members locked up, with an ongoing online petition she's maintained for a while now.