During an interview with Extra, the 25-year-old gushed over her little brother's pop star girlfriend.
"I think they're adorable," Katherine said of the cute couple. "If my brother's happy, I'm happy and that's all that matters to me. You just want your sibling to be happy, if they're with someone who makes them happy and in a happy healthy relationship then I'm happy."
Speaking of the "adorable" lovebirds, a source tells E! News that Miley and Patrick's relationship is getting ‘'very serious."