Is learning to see aura's a sin?

It’s folly.

if this is magic or spiritisim no its not ok
http://www.remnantofgod.org/

only when staring at naked people

No, it’s not a sin.

Aura’s like in a feeling around a person?
That would be a gift of the Holy Spirit. Aura’s can be sensed by our 7th sense. That would be from our spiritual soul. To see a vision of them, it needs to be God given.
I have felt demons or Godly Angels, but not very often see a clear vision of them. God does let me see a vision of demons to get me to recognize it & take authority over it by the power of Jesus Name.
I have also felt the Godly Angels wings & it would be a sense of where the wing is. They don’t like to let you know what they exactly look like.
Today, my Guardian Angels described himself. About 7’7″+ tall. Golden blond hair. Golden tan skin. Golden brown eyes & white sparkle. White wings & a white robe & Golden sandals. But he won’t let me see a vision of him, but does talk to me much. But he has given me a vision of an impression of a smile, etc.
I would suggest that you read Corinthians about Gods prophetic gifts. Discernment of spiritual is one way the Holy Spirit teaches us. Comparing spiritual with spiritual.