Second Impressions

If you counted the first meeting as two people sharing the same space and time for a moment, then Sirius and Marlene met when they were seven. They brushed past each other in Diagon Alley, Marlene holding Elspeth's hand as they shopped while Sirius held firmly to Walburga's robes. Neither turned to look at the other, so any reasonable person would not count that as a first meeting.