Is it ok for a 15 year old nephew to tickle his aunty's feet in a slow and smooth way?

That seems normal. I have friends whose aunts/moms make them tickle their legs/feet like that just because it's relaxing to them (after a long day or whatever). Sort of like how a massage is relaxing. It's not meant to be sexual, although I could see where you'd mistake it to be lol. :)