Still, the Kardashian family is worried about Rob's new relationship
, a source close to Rob exclusively told ET last week -- and it's not hard to see why, given Chyna's complicated history with the family
. The "Rack City" music video star used to be close friends with Kim Kardashian, but the friendship cooled after Kim's 18-year-old sister, Kylie Jenner, started dating 26-year-old rapper Tyga. Tyga is Chyna's ex-fiance, and the two have a 3-year-old son together, King Cairo.