Fallen Angels...
Other than satan and the 1/3 of angels who followed him, there are no other mention of "fallen" angels. So is it possible for angels to sin? Well based on the evidence, that there has already been 1/3rd of them who have already fell, it would imply that God has not only given "free will" to mankind but to angels as well. Based on this logic, i would have to say yes. Angels can sin. However it brings to light an interesting theological debate. Like Judas, how is it even possible to be in the presence of our Lord and yet choose to turn away?
So although there are no specific scripture to support the premise, i believe that one can deduse from existant scripture a logical argument to support the notion that angels and sin. Thank you for the interesting conversation.
"Grace and Peace be multiplied to you through the knowledge of Gof and of our Lord Jesus Christ." II Peter 1:2