I think OP and all of the commentors are westerners and are totally missing something - she's Southeast Asian. Maybe she's not an immigrant but she might be, or her parents are.

From the wikipedia page on swatstikas:

"However, the swastika remains a symbol of good luck and prosperity in Hindu, Buddhist and Jain countries such as Nepal, India, Mongolia, Sri Lanka, China and Japan. It is also commonly used in Hindu marriage ceremonies and Diwali celebrations."

If you ever go to these countries you will see swatstikas like, everywhere. It's an ancient symbol that was appropriated by the Nazis that has an entirely different context and meaning in Southeast Asia than it does in the west.

This is probably the equivalent to an Irish girl getting a clover tattoo to her. I think OP and all you other commenters need to cool off and remember that just like Westerners can be extremely ignorant of genocides and atrocities committed in the Eastern world now and throughout history, so the same can be said for the reverse. Even if she's grown up in the western world, she might not fully grasp that the swatstika is seen as exclusively a symbol of the Nazis here. She's probably ignorant of the gravity of it's symbol here, but are any of you all really in a place to judge her when you're just as ignorant of what the symbol means in Asia?

All that doesn't mean the tattoo should be exposed during the wedding. OP is western and it's a western wedding and it's not appropriate. I think it's very reasonable to ask her to have it covered up for the wedding and if she has a problem with that then kick her out.

All I'm saying is have some restraint before jumping to judging her as a Nazi, she's almost certainly not.