Thor did not know Gamora's sister, though he'd learned that there were a number of "children" Thanos claimed as his own. Another was there, in town? So long as they didn't cling to Thanos' ideals then it wouldn't be a problem. The Gamora he'd met certainly didn't, and he'd not judged her even for a moment when it'd been revealed to him who she was. Why would he? He knew what it was like to have family connections that were less than honorable, and he'd told her as much.
"I attempted to cause the death of Thanos on Midgard," he said. "I missed." Thor hadn't seen her since he'd left with Tree and the sweet rabbit in space. It sounded as though she'd seen more. Thor had learned from Natasha that he'd killed Thanos, but didn't think that was what Gamora was talking about. Though she seemed to know who he was, it didn't really fit that it'd taken her that long to put his face to not even a name, just a title.
"I have met you, in my time," he said. "I spent time with you in space, though it was brief." Thor took the coffee black and started to drink it. "This is very good."