I think this question all makes no sense.
Becuase it is all personal opinion to decide if something is a sin to you.


Its up to you to decide whether what your doing is right or wrong and justified or not.
If you think its only a dream and its okay, then go for it.
If you think its wrong to do it in a dream, don't do it.

I myself have no problem in doing something bad while lucid dreaming.
My philosophy, LD's are for your enjoyment, its not like the 'Dream Police' are going to lock you up.
So what the hell do you have to loose?
Its not real life, so you haven't hurt any real people.
And if no one is hurt, then its okay for me.