Sebastian Telfair's Sister Arrested by FBI for Threatening NBA Star's Estranged Wife

Sebastian Telfair's sister has been arrested by the FBI for allegedly making violent threats to the ex-NBA star's estranged wife ... TMZ Sports has learned.

We broke the story ... Samantha Telfair had gone to police in late April claiming she was getting frightening phone calls from Octavia Telfair, who threatened to "kill my kids and cut my face" all because she testified in Sebastian's NYC gun case.

During Sebastian's trial, Samantha told prosecutors she had personally witnessed the former basketball star in possession of multiple weapons, including the gas-operated submachine gun he was allegedly carrying when he was arrested in June 2017.

Apparently, Sebastian's sister, Octavia, felt Samantha's testimony was treacherous -- so she allegedly called Samantha more than 50 times on the phone to express her discontent.

The LAPD and NYPD were investigating -- since Samantha lives in L.A. and Octavia lives in New York -- but the case was kicked to the FBI, which ultimately decided to arrest Octavia.

As we previously reported, Telfair -- a 33-year-old former high school phenom who spent 12 seasons in the NBA -- was arrested in June 2017 in Brooklyn when cops found several weapons during a traffic stop.