People were very casual about this. Jane Bennet refers to her nieces and nephews who are in fact her cousins (ie children of her mother's brother).

In everyday usage, there often seems to have been a usage that young children would call their adult relations uncle and aunt, no matter what the strict genealogy books would say. If you're looking up a generation, you call your relations uncle and aunt; if you're looking down you call them niece and nephew, and cousin is the all-purpose word for everything in between.