Sofia also posted similar photo on her Instagram Story, writing "Support team."
While Lionel was clearly happy to welcome his daughter to the set, he played coy with ET's Lauren Zima as to who he thinks will win American Idol this season.
"I will not say. I will not say. I want to give it an even playing field, but I will say to you that they are so talented," he mused. "The nerves got them tonight, I agree with that. It was just a matter of let's just get past this and get the voting done because I want people to vote."
The finale of American Idol airs Monday on ABC at 8 p.m. ET.
As for what Lionel thinks of his daughter dating Disick, here's what he said of the relationship earlier this year: