Since a sinning Holy One is a self-contradiction, it's not a 'thing' and therefore it's no limitation to God's power not to be able to sin. One way to look at it is that God's nature provides Him a range of things He can cause and that range doesn't include sin. Another way to look at it is that desires are a necessary, but insufficient, condition for choices. You have to desire something to be able to choose it, but if you desire it, you still may choose something else (that you also desire). God only has good desires, so He cannot sin.