Is 'aunty' not considered an acceptable translation of 'moster'? Perhaps it's just a colloquialism where im from.
It is a colloquialism, which is exactly why it's not accepted. "Moster" doesn't have any special colloquial connotation in Swedish, so you shouldn't translate it as one when just "aunt" is a perfectly good translation.
In Australia at least it is not considered a colloquialism, it's the other way around. Aunty is the common usage, Aunt is.. kinda weird.
Why is "att" not needed here, with the object following the infinitive verb?
Att is not used with modals: https://www.duolingo.com/comment/7075383