Buffy remembers her mom telling her that it would be her job to help watch out for Dawn.
And she did. Holding Dawn when she skinned her knee in the playground. Hiding Dawn's eyes when the scary parts of the movie came on. Teaching Dawn how to use the curling iron for her elementary school graduation.
But none of it happened.
And if there was never a Dawn, then this Buffy never existed either.
She mourns for a life she doesn't remember even as she knows it doesn't matter. This Buffy loves her sister and will go on protecting her.