Your little sister had been working in a house of pleasure in Lys before becoming King Maegor's seventh wife. You never approved of this, you had begged her not to do this, you knew guys like maegor wouldn't fall for girls like her. But she hadn't listened.
But she was never treated like Maegor's queen, just like you think. She fled to Westeros because she was in love with Maegor, but the only reason Maegor made her his wife was because of your sister's promise to bear him a baby. And during her time as Maegor's wife, she was seen as nothing more than a baby machine.
She had gotten pregnant three times, but all three times had failed. Tyanna, one of Maegor's previous wives, had poisoned her. She was probably jealous of your sister's successful pregnancy. Because the letters your sister sent to you several times described the way Tyanna treated her. And in her last letter... She also talked about how the men in the court treated her. They still treated her like the girl she was in Essos. In her last letter, she wrote to you that she could no longer tolerate their touching her. And on the letter, stained with tears, it said that Maegor no longer cared about anything that was done to her.
But you hadn't answered any of her letters. You still hadn't forgave her for running away to Westeros after you had told her this would happen. And finally the news came to you that your sister had hanged herself.
You were in deep mourning, and the only thing that kept your grief from turning into a consuming depression was your consuming desire for revenge. You infiltrated the Red Keep as a servant and tried to gain King Maegor's favor for weeks. And you did it.
Your goal was to kill him. But Maegor wasn't that stupid either. He saw the birthmark on your back, your sister had the same mark, she had told him about these birthmarks when she was alive. Still, he acted like didn't understand and let you come to his room. He was sitting on the chair with a glass of wine in his hand, grinning as he watched your stupidy.