Theory: they most likely are childhood friends, so it's a possibility that he truly sees her as a childhood friend, if not then according to the previous timeline wouldnt he already be somewhat infatuated with leisha. If that's not the case then i guess in the og timeline they met, fell in love, and the only reason he refers to her as a sister is bc it would be treason (plus may ruin leisha's reputation) to refer to her as his ex lover. Anyways thank you so much for the fast releases on this series!!! Hope you stay safe!