She was about 18.
So it is very possible that she could have been interned without being a dual citizen as long as her parents were.
But Japan was planting colonies in the Americas. The US, Canada, Peru all places they put colonies with the idea of gaining a foothold and then taking over.
This was not a "pie in the sky" fear but something that had been ongoing for several decades before the war started. In Peru for example if you look at the leaders of the Japanese colonies there you will find they were all native born Japanese with their first loyalty to Japan.
In fact Peru deported many of their Peruvian born Japanese to the US for internment as did a number of South American countries.
You can say they were all suffering from hysteria.
Or you could say that when people come into your country and live in enclaves, send their children to be educated back in the "old country", do not bother to learn your language, take their orders from their former country and make no attempt to integrate that they might be regarded as a threat.