Oliviakins: In the US aunt is probably the most common term for the sister of a parent. In most parts of the US it's pronounced to rhyme with 'want', and as a form of address it's quite often lengthened to "auntie", while in the south it's pronounced 'ant' like the pesky little critter we're all familiar with.
That's interesting, because in the US aunt is very casual when talking about your aunt and auntie is really only used by small kids or when addressing your aunt. And I'm pretty sure all English courses on Duo are American-English based, and we would never say "my auntie(s)" to someone, so that's probably why. But it should be accepted.