Haaaaa, sins are what gets to me. I will put up the notion that I'm really interested, I've a fondness for 'evil' characters, and the idea of a sin as a superpower seems really interesting. Now, as someone who is into psychology and done his research on the seven sins, I'd like to say that Pride is the one I'm most intrigued by as a usage for power. 

How to use such a weirdly defined sin though... The idea I'm having is the ability to change appearance in a way that other people idolize, the essence of vanity is being what everyone desires, at of course a cost. The longer the character would use the power ( not mentioning he or she yet ), it would slowly tear and tain the sanity of the person, slowly losing the mental image of how he/she really looked like. It's a really weird draw-back, but I'd imagine that someone who shifts so often in appearance slowly loses the grip on the reality at hand. That and it would definitly affect the character's self-esteem in a negative way after periods of usage. 
It's a small draft of what I have in mind, I don't know for sure yet, just wondering what you would think.