"Good morning, Auntie."
If you're speaking to a person who's older or in a higher position you should speak formally.
I'd say it depends on your relationship with them. Age isn't the only factor and if you are close to someone, you generally speak informally.
I would think that if you're calling them Auntie, you're close enough to be less formal
As a language instructor for Japanese we usually teach our students that the Japanese culture places great importance on respect (as with most asian countries). Hence, when talking with people who are of higher authority/older than you, we tell them that they should use おはようございます (good morning) and not just おはよう.
Regardless if your aunt is of the same age as you or even younger. That person is still your aunt and in Japan, teachers usually speak formal to their students as well (even though they are older of course).
Of course there are some who don't really speak in a formal manner with their relatives as they are already quite close. But, you also need to take note of the cultural difference between Western and Asian cultures. While it is normal for people living int the US for example to call their Aunts/Uncles, etc. by their first names, that is not the same for the Asian culture. Again, the asian culture places great importance in showing respect for their elders.
For those saying "what if the aunt is younger than you" either way I think that you should still greet them formally at first (and you can change your tone to your casual speech should the other person state first that you don't need to be formal (you never assume it's alright to be informal just because you're the same age or just because the other person is younger). Also, when people say Aunt, more often than not they are usually older than you (aunts who are at the same age as you are or younger are usually a rare case so it's just best to go with common sense here).
I think part of the confusion is the duo keeps using "Auntie" which is super informal. Using "Auntie" really implies informality.
おはよう is usually written in hiragana. (The kanji for おばさん is pretty common, though.)