We saw Leia tell Han she was Luke's sister.
Claudia Gray made it clear he knew Vader was her/their father from the start.
We did not see her tell Han that part on screen.
Claudia also mentioned they had a conversation on Endor while laying on soft moss with the smell of cedars and pines surrounding them in BLOODLINE...
The Ewok's knew how to help take down an Empire, but they really knew how to throw a victory party. It had already been a few hours and the revelry showed no signs of slowing down. Lando and his crew were happily engaged in conversation. Luke was enjoying relaxing to the side, watching.
Han couldn't help but notice Leia had gotten very quiet as she laid against him, her head resting on his shoulder.
"You wanna go back to the hut?" he asked quietly, hoping to get her to open up. He knew something was wrong. She'd be more likely to tell him what was going on if they were alone.
She nodded and stood eagerly, offering her hand to help him to his feet. He smiled up at her, laced his fingers with hers and they began walking slowly down the path to the suspension bridge that led to the their temporary hut. Han got an idea and led her off the path to a little clearing in the forest he noticed during the battle. The air had cooled considerably once the suns went down. A breeze was rusting their brown hair and filling their senses with the smell of cedar and pine. There was a soft patch of harmless moss under a very impressive cedar tree. He eased them down onto it.
"Talk to me," he encouraged in a soft whisper, his lips brushing her ear as he spoke.
He felt her tense for a second, but she gave him a gentle squeeze and snuggled closer against him. It amazed her how instinctive he was about her, knowing her needs, wanting to help. It would not be fair to keep it from him. He had to know who he was in love with, made love to for the first time in the Falcon on the way to Bespin. He'd run the other way as fast as he could and never look back. And she'd understand.
"Luke had a lot to say," she sighed, trying to figure out where to begin.
"How is he?" Han asked quietly, his thumb beginning to stroke her shoulder, encouraging her, comforting her.
"He's going through a lot. He's decided to go off and become a Jedi Master, so... He'll be gone. We won't see much of him ever again," she said sadly. "He said I have the power, too. The Force."
She raised her head and settled on her elbow. She watched his hazel eyes. He swallowed gently and nodded. "And you'll be leaving, too," he added sadly, bowing his head. Just like when he assumed she "loved" Luke, he assumed the worst. No doubt that came from the hard, lonely life he'd led.
"I'm leaving with you," she assured him. "I told Luke I didn't want that life because that would mean I couldn't have a life with you. And I want you. I want a life with you... If you'll still have me after what I have to tell you," she said cryptically.
He furrowed his brow and then smiled. Tears sparkled in the moonlight. No one had ever given anything up for him, let alone family and power. She really did love him. He tenderly took her face in his hands. "Just tell me, sweetheart."
She bowed her head, unable to bare seeing the look in his eyes, and decided to do just that: Tell him. "You know Luke and I are twins... Our father ... is Darth Vader," she whispered, her voice breaking, her tears falling swiftly and earnestly. Shame filled her. Fear of his reaction and subsequent leaving was making her tremble. After the briefest of moments, she had her answer, finding herself held tightly in his arms. He was staying by her side, supporting her. "No one else knows but you... For now. You won't be hurt by it, I won't allow it," she vowed, daring to look up into his eyes again. She had the bad feeling that sometime down the road, it would come back to haunt them in a terrible, devastating way. But for now,
He leaned his forehead against hers, his thumbs tenderly wiping the bitter tears from her soft cheeks. She knew she would remember the look of pure true love in his tear filled eyes for the rest of her life. He must have sensed her turmoil.
"You think I'm gonna walk, don'tcha? Well I'm not," he assured her, pressing his lips to hers in a deep, passionate kiss. She melted against him, relived and happy, full of love. After a few minutes, they broke the kiss. He stood and pulled her up into his arms, lifting her off the ground and adjusting her weight into a carry position. Her waist length brown hair swung and she giggled, sniffling the last of her tears away. "Let's go back to our hut and, uh... set off our own private fireworks," he smirked, his lips brushing hers as he spoke.
"OK, hotshot," she sighed, smiling against his lips, knowing exactly what he intended, his hand already splayed on her thigh.