Well before choosing any option, I'd wait another couple weeks, maybe the skin will change a bit more, who knows.
If that doesn't help, go with checking DNA behind her back, provided you live in a country where that is legal. In some countries, you must have agreement of both parent to have DNA tested, so... If not, before you confront her, you might wanna consult with a medical specialist about the odds of that happening. Sometimes people get ginger children, even though parents are both dark-haired ( that is actually possible ) and some other "anomalies". Maybe you are the lucky one?
12 years is a long time to fuck it up so quickly, don't make any rash decisions and don't be too hasty to drop accusations on her. Think this through and do everything that you can, and only if everything else fails, try to have a suggestive talk with her, see what she thinks.