According to Hollywood Life, Kylie has been leaning on Rob for support following her breakup with T-Raww, and Rob wants to do everything in his power to protect her. He's concerned that Tyga will "cause problems" for Kylie now that they're over by constantly pressuring her to get back together with him.
As a result, Rob went into full-on big brother mode, warning Tyga to stay away from Kylie.
"He's had enough of Tyga's games and has made that very clear to Tyga and told him to pump his breaks," a source close to the family told Hollywood Life. "Rob doesn't want Tyga bombarding his sister with phone calls, showing up at her house, sending her gifts, none of that. He's explained to Tyga that Kylie needs time to be alone and to breathe."
"[Rob] demanded Tyga respect his wishes," the source continued. "Rob's very loving and he gets along well with everyone, but if you cross him, especially by hurting those close to him, there will be problems. Tyga's walking a very thin line."
This is just further proof that Rob has always been the best Kardashian (sorry, Khloe).