Complex sentence: Margaery first appears in the second season, following her marriage to Renly; she is well aware that her marriage is a political one, and displays pragmatism regarding Renly's homosexuality and his relationship with her brother Loras.
Split into simpler sentences: Margaery first appears in the second season, following her marriage to Renly. Margaery's season 2 appearances establish that she is well aware that her marriage is a political one, and she displays pragmatism regarding Renly's homosexuality and his relationship with her brother Loras.