The Importance of Being Earnest2002

Algy:
But why does your aunt call you her uncle?

[Reading cigarette case]

Algy:
"From little Cecily, with her fondest love to her dear Uncle Jack." There is no objection, I admit, to an aunt being a small aunt, but why an aunt, no matter what her size may be, should call her own nephew her uncle, I can't quite make out.