Kylie Jenner says she expressed her concerns to her brother when he got into a relationship with Blac Chyna … because Kylie’s own ex, Tyga, had warned Chyna was violent.

Kylie took the stand Monday in the Chyna v. Kardashian trial … and Chyna’s lawyer asked her about her relationship with Chyna. Kylie says she wouldn’t have ever considered Chyna a close friend, but says Tyga — who has a kid with Chyna — once showed her a 6-inch long scar on his arm, claiming it was a knife wound at the hands of Chyna.

Chyna’s attorney continued questioning, and Kylie revealed Tyga told her Chyna abused drugs and alcohol — and they were to blame for the violent knife attack.

Kylie says she brought her concerns about Chyna’s behavior to Rob Kardashian, but says Rob told her he was going to continue to date Chyna. Kylie said in court, “I felt it was my duty to express my concerns, but ultimately it was up to him.”

This is the second week of testimony in the case … we’ve heard from Kris, Chyna and now Kylie. Kim was not in court Monday … as we reported, she’s in D.C. with Pete Davidson.

Last week we also saw Chyna’s mom, Tokyo Toni, get banned from the courtroom for bashing the Kardashian family and then she seemingly threatened the judge … though she later told our camera the threat was aimed at Kris Jenner.