Brandon Blackstock's romance with Kelly Clarkson's former assistant Brittney Marie Jones was a "big part" of his life before his death at age 48. "Brandon and Brittney were in a relationship, and his family was very happy and supportive," an insider exclusively tells Us Weekly. "His children spent a lot of time with her and had grown close to her." (Blackstock shared two children with ex-wife Clarkson: daughter River, 11, and son Remington, 9. He also shared daughter Savannah, 23, and son Seth, 18, with first wife Melissa Ashworth.