Complex sentence: When they arrive at the house, he meets his Uncle Sora and discovers that the girl directing him is really his cousin and that day was her 16th birthday and the family will be having a party together.
Split into simpler sentences: When they arrive at the house, he meets his Uncle Sora and discovers that the girl directing him is really his cousin. He finds that this is her 16th birthday, and the family will be having a party together.