I don't really see BW2 and XY happening so close because that would make X and Y older than Lack-Two and Whi-Two and that would feel pretty weird to me. Just because 'Colress' goes to Alola at the end of BW2 doesn't mean the arc takes place immediately afterwards.
Y's birthday is in February, which means she's almost 13 in the epilogue. If that was before the March in BW2, that'd make her older than both BW2 kids. With X it depends on whether you consider him 12 in the main arc in October or in the epilogue, he could be the youngest or the oldest. IIRC Pokespedia wasn't consistent on whether the ages were for the beginning or for the end of the arc.