I agree. I tried that before, in this sentence or another, and it was rejected. I did report it, but I don't know if it is now accepted anywhere. "Aunt" is a bit formal for me and I would normally say "auntie".
I found it pretty heard not to hear the verbs as 'rithim' and 'siúlam' - the latter not quite as bad with a bit concentration. It just didn't seem to make too much sense - "I run my aunt and walk your uncle".
Irish (and English in Ireland) doesn't really make that distinction. Aintín is a diminutive. The dictionary suggests that some people use aint instead of or as well as aintín, but there is no general formal/informal distinction.