According to my signing vocab book:
Aunty - bent index and third fingers tap your chin. The picture shows the two fingers slightly apart. Kind of like doing a V sign on your chin, but with the fingers bent. My sister always laughs when I do the aunty sign to ds!!
Uncle - hold one hand up in front of you palm facing in. The index finger of your other hand brushes twice on the back of the little finger on the hand you are holding up.
Blimey these are hard to describe! No wonder there are pictures in the book. Perhaps someone will be able to give a better description for you megglevach.