Perhaps, just perhaps, now might be a good time to mention the best selling book of all time: The Bible.
Dan's mother was not a big fan of the Bible, what with it saying, "Thou shalt not suffer a witch to live," and other such things. As such neither Dan nor Kat had ever really read the Bible. They knew nothing of Genesis chapter six, and were therefore totally unaware that certain angels once interbred with humans creating Nephilim.
This meant that neither of them had any idea where they had come from. Nephilim was in their blood, and the blood of all witches. It had been diluted and confused until it was normally only accessible to the daughters of the Nephilim via arcane rituals.
But every so often, by some quirk of bloodlines and accident of statistics, one of the boy children had just the right mix of human an Nephilim. Even then nothing usually happened, it would take a severe shock to the system for one of the sons of the Nephilim to regain contact with the potential inside of him.
A shock to the system like, say, being cursed to turn into an animal. Even then Dan probably wouldn't have reverted to full Nephilite, but something about the feathers recalled his distant ancestors.
The feathers on Dan's chest and back receded and he breathed a sigh of relief. Then he felt something in his back. His shirt felt tighter and his back very uncomfortable. It was as if part of his back was being crushed. He rolled onto his side, which relieved some of his discomfort but his shirt was still too tight. It was as if something was inside it with him and there wasn't enough space for both of them.
He struggled out of his shirt, finally ripping it off over his head in a surge of effort, and then relaxed slightly as his wings were no longer uncomfortably constrained.
His ... wings?
Yes! Wings! Feathered wings, mostly blue with some white and black feathers too. They reminded him of bluejay wings. Dan wondered how this was possible, and then knew the answer.
Suddenly Dan knew all about the Bible, and the Nephilim, and Noah's flood, and every language ever spoken, and magic and ... this was cool.
Coolest of all was that he knew that Nephilim were immune to magic. There was nothing his sister could do to him. There was a lot he could do to her. Dan realized he had a choice to make.
Barring a massive global flood, angelic intervention, or another Nephilite showing up he could do whatever he wanted. Did he want to good things, or did he want to have some less than noble fun?