What is a sin to a wiccan?

First, let's define a Sin:

A sin is a Judeo-Christian concept.

A sin is one of the prohibited acts set forth on a holy document.

A sin could be forgiven by the Highest Force, or otherwise irreversible.

In this context, Wiccans don't believe on the concept of Sin. Wiccans believe the law of three, where, whatever you do, Good or Bad, comes back to you three times. The Wiccan Rede says "An it harms none, Do as thou Will".

Further, there are a large number of Wiccan laws set forth on the Gardnerian Book of Shadows, including the oaths of (mainly focusing on) secrecy and prohibiting a witch to perform magic for the purpose of entertainment. Those are not like the rules in the Judeo-Christian tradition, those laws are meant to be protected by each individual for the safety of the whole pagan community. And Witches believe that if those laws are broken (endangering the whole pagan community), the curse of the old ones will be upon them, and that's it.

In Wiccan belief, you yourself has the full responsibility of whatever you do. Once done, nothing is forgiven. There is no higher power that can forgive you. Everyone has to face the consequences of what they do, good or bad. However, if one realize that they have done a mistake and learn from that, they can always do good to the world and try their best to clean their mess. Learning from the past mistakes and cleaning your own mess is the Wiccan way of getting rid of the Karma.

Karma always follows the intention. Anything that happens around you without your intention is not counted as Karma. Good intention brings Good karma, and Bad intentions brings Bad karma.