Yes, he's gone too far to turn back now.

For a moment Billy considered resisting, drawing a line in the sand, calling this humiliation a step too far. He considered fighting Anna, throwing her off of him, beating her and dominating her. Even though she was already up 2-0 in their matches, Billy knew he was the stronger sibling and he knew he could beat her in a fair fight.

Those thoughts came to an abrupt end as Anna tensed her thighs ever so slightly, pressing on Billy's chest and ribs. He looked up at her back, admiring her curvy figure. And the voice in his head was now saying, they already HAD a fair fight, and Anna had kicked Billy's ass. There was no reason to believe he'd do better in another match against her.

As Anna straddled him and lazily stroked his cock with her hand, the realizations kept hitting Billy. Anna was smarter than him, and had better conditioning. That meant that she would improve faster than he would. Even if he focused entirely on boxing, she might still outpace him. Fuck, he probably missed his best chances to beat her already.

The possibility that he would never beat Anna in boxing, that he would never get **** or even the score with her, started to take root in his mind and gradually become a certainty.

Anna wriggled her butt expectantly, just a foot or two above and in front of Billy's face. "Well, are you going to kiss it or what?" she asked. He couldn't tell how serious she was.

But it didn't matter, because something broke inside Billy. The combination of Anna's insane, unexpected hotness and her equally unexpected boxing prowess had blown his mind and broken his spirit. He leaned up, doing most of a sit-up and bringing his lips to Anna's bare butt. He gave her a long, passionate kiss right on one of her cheeks.

He'd already forgotten about the fact that he was doing this in exchange for a blow job; he just wanted to pay tribute to his goddess of a sister, humiliating though it might have been. But the instant her lips encircled the head of his cock again, he moaned and tensed as the pleasure returned to the forefront of his mind.

He kissed her ass again and again, eventually starting to lick it, going into a frenzy, anything to get her to continue her scintillating blow job. Where had she learned to do this? Or was she just a natural, like she seemed to be at everything else?

Billy's abs started to burn as he tried to hold his body in the sit-up position without his hands and arms to support him. He felt his strength fading fast, but he couldn't relax, because then he wouldn't be able to kiss Anna's ass, and she would stop giving him the best head of his life. So he pushed himself to his limit, continuing to lavish her with kisses even as his body started to shake and tremble.

Anna must have sensed that Billy was running out of energy, so she stepped up her game and swirled her tongue around Billy's cock in just the right way to push him over the edge. Billy started shaking for real, and came explosively right into Anna's mouth. He gasped and flopped back onto the bed as Anna rode him and swallowed his cum.

After Billy finally went limp, Anna spun around on him and faced him, pinning him down with her hands on his shoulders.

"How did you like THAT?" she asked proudly.

"It was...amazing," Billy said hesitantly. It had been humiliating, too, but he didn't want to openly admit that.

"Are you going to challenge me again?" she asked curiously.

Billy couldn't meet her eyes. "No," he said. He knew now that there was no point.

Anna slapped him unexpectedly, rattling his brains. A slap shouldn't do that much damage, but in Billy's weakened state, he wasn't tough enough to withstand it.

"Wrong answer," she said. "That's boring. You're going to challenge me every day. And my guess is, you're going to lose every day. If you don't challenge me, I'll challenge you...in front of Mom. That way you'll have to accept!" she said gleefully.

Billy groaned. She had him backed into a corner. He knew he was going to agree, but maybe he could get something out of it.

"Will you give me any more head?" he asked, hopefully...and a bit pathetically.

Anna giggled. "Sure, if you don't embarrass yourself too much! So, in other words, probably not!" She laughed at her own joke, even slapped her knee with the hilarity of it.

Billy didn't find it funny. He was terrified of the new reality he was stepping into...and also incredibly turned on. He felt lucky that Anna was paying this kind of attention to him, but nervous about the fact that she clearly owned him now.

"Alright, bro," she said, slapping both palms on his chest. "Get some sleep, you're going to need it!"