Another twist in the murder case of Queens dentist Daniel Malakov: The sister of his ex-wife - who remains in jail for plotting to kill Malakov - was arrested for threatening Malakov's brother.

Natella Natanova was arrested yesterday after apparently approaching Gavriel Malakov in Queens and saying, "You do know if you talk, you will be the next to go," on Monday morning. The brother is a potential prosecution witness and Queens DA Richard Brown said, "Prospective witnesses must be protected from outside interference that might affect their testimony."

Her sister, Mazultov Borukhova (pictured), who had been locked in a bitter custody battle with Daniel Malakov for their young daughter Michelle, was arrested last month. The Queens DA's office alleges she paid a relative $19,000 (and made 90+ calls to the relatives in a short period leading up to the murder) to kill her ex near a Queens playground.

When he heard of his ex-sister-in-law's arrest in February, Gavriel Malakov had said, "I cannot believe you think America is so stupid, so dumb that nobody is ever going to get caught." Now, he told the Daily News, "It's a bitter situation for me because even though they are arrested, I can't bring back my brother." He added he feels the whole Borukhova family was involved in the murder.