Sirius had been surprised when Regulus invited him to dinner at his flat. He had decided Regulus would never trust him that much. With some sense of occasion, he had considered what to wear as a guest in his brother's home. Regulus would, of course, wear robes, and Harry was unlikely to change from jeans. With a sense of compromise, he chose a comfortable linen suit in cerulean blue and darker gold -- not formal, except in the beach sense -- and paired it with tan leather sandals. It wasn't that warm, of course, but that was what charms were for.
"Hello, Regulus," he said, as his brother opened the door. Right on the robes, wasn't I? A glance inside was enough to justify a "Nice place," so he said it.
It only belatedly occurred to him that he should have waited until he was shown around, and perhaps he was a little bit nervous.