Complex sentence: He is excited for his grandpa to get married as he anticipates finally getting an uncle to hang out with until he realizes his "Uncle Chris" is a bratty 12 year old kid, but Johnny remains determined to get Chris to like him.
Split into simpler sentences: He is excited for his grandpa to get married as he anticipates finally getting an uncle to hang out with. He's excited to hang out with his new uncle until he realizes his "Uncle Chris" is a bratty 12 year old kid, but Johnny remains determined to get Chris to like him.