Oliver Hudson, 45, and Kate Hudson, 42, made sure to post a 40th birthday tribute for Britney Spears! The brother-sister duo sang “Happy Birthday” for the pop star, with Oliver taking the lead. “Happy birthday — ooh, happy birthday to you….happy birthday to Britney f—ing Spears,” the Nashville alum crooned in the video posted by Brit on Friday, Dec. 3, with some assistance from Kate’s adorable 3-year-old daughter Rani Rose on the backup vocals!