He doesn't care

Adam didn't care what Sarah was saying. He was already transfixed on her cock, and started to suck her off. He heard Sarah’s surprised moan; that made him even more eager to suck her off. He did not know why he was doing it, but it felt… right. Having his mouth filled by his sister’s monster. Having it inside his mouth, letting him taste it.

He wanted more. He desired more. Adam tried to deepthroat her with little luck, but Sarah liked the idea. She put her hands on his head, and started to pull him down on her cock. Feeling Adam’s lips close around her member, while his tongue did the best to taste her whole dick.

“I didn't know you were into cocks this much. It might answer why you never had any real contact with any woman,” Sara said. Adam did not reacted. He just tried to get more of her delicious meat inside his mouth. She did not like that, but the way he was sucking her made up for it. “Show me how good a cocksucker you are!”

Adam was hard pressed to resist his gag reflex but did his best to taste her cock and her precum, and at the end maybe even her cum. The idea was strangely inviting. Adam did not know why he wanted her this badly, and even less why he wanted her this way, but why resist something that felt just right?