To do aunt, make a fist with your thumb sticking out near your face, at top of the fist. Leading with the thumb, move your hand in a small circle near your cheek (but not touching it).
Expect your baby to start with a much simpler version of aunt. Look for any type of fist near their cheek.
Aunt is a real crowd pleaser. This will melt the heart of your sister, even that shrew of a sister-in-law! We use aunt very widely to describe any close adult female friend. Use aunt when you first introduce an adult female friend or relative to your baby, then again in a lull in the conversation, and one final time when you leave. "Look, it's Aunt Lauren," then, "It's Aunt Lauren", and finally, "Say bye-bye to Aunt Lauren!" Get the aunt to join in on the act.
Stick a picture of your baby’s aunt over this stock photo so she recognizes the picture as her aunt. If your baby has a few aunts, make a series of flash cards with different photos of each of the aunts.