Quill didn't remember Thor mentioning his brother. Quill remembered not paying a lot of attention to Thor when he woke up in the ship because everyone else had been obsessed with him. He almost apologized for Loki, but then Thor continued to speak.

He squinted again while he thought. That doesn't sound right. Why doesn't it sound right?

He thought about back home. Six stones

Clarity filled the once-muddled expression of Peter Quill. He lifted his head and looked at Thor. Quill suddenly looked a lot more sober.

"That's impossible." He said. "He didn't have six on Titan."