Luke didn't seem to recognize Rey at the end; just that he recognized his old lightsaber that he lost at Bespin in Empire Strikes Back (ESB). It's not apparent that Luke ever met Rey, so I don't think she's his former Padawan or that she's related to Luke or Leia. If Luke were indeed her father, that should have been made clear at the end, but it's not, so we're left with doubts for two years. I think the premise that she is Luke's daughter is a misdirection by the writers who want us to assume that. For my theory, I'm assuming and hoping that there's a fairly unexpected, but hinted or foreshadowed plot twist regarding her mystery for the sake of a good plot twist in Episode VIII or IX. I think the writers intend for the plot twist to be a shocking revelation about who Rey really is.