I think if she wants to, and you want to then just do it. I was raised Catholic, but just couldn't live by man made rules. But that's just me.

If you want to play by their rules, then yes I suppose it's a sin, but imho I just can't see why it would be, but whatever lol.

Relax, have a good time, and be nice and reciprocate to her, and try not to think of the religious repurcussions cause that'll just rent space in your head and well you want to focus and enjoy