My uncle just got married and I'm falling for his new wife's niece! I'm not sure if this means we are now second cousins or something! Any help would be appriciated

You may be related by marriage, but not by blood. Attractions to others is natural, and human, but what counts is how you handle yourself, so don't get carried away, and do something to cause you, or her some embarrassment.

Try to always act appropriately.