Though I doubt that the OP is honest this happened a lot historically. Mostly in royalty but I imagine that if incest was approved of for royalty it was probably practiced sometimes by the masses as well. King Tut married his half sister, Mausolus married his full blood sister for instance. So, you're not alone.

Basically you have two choices, talk to her about it, or don't talk to her about it. You need to decide which is the better choice.