A few other circumstantial things lightly hint at the twins not being typically American:

1. Obliviousness to local laws and oversight (and the combination of severity of punishment + likelihood of getting caught); most Americans of their intelligence would be aware, and would not engage in the sort of hijinks they did.

2. Working with sibling (anecdotal, but seems slightly more common among immigrant families than locals, which would make sense since, on average, immigrants have fewer local connections than locals so the likelihood of working with siblings increases)

3. Loyalty to family (evidenced through the brazenness in the way they helped each other in criminal acts without a second thought). Americans, on average, are more individualist and hesitate more when asked by family to do something criminal

4. A lot of immigrants eventually adopt anglicised names, which neither of these two did

If a detective looked at these facts, they'd keep an open mind as there's nothing definitive above, but it would be equally ignorant to ignore the circumstantial evidence.

Having said all this, do we care where they're from? (unless it's a potential case of foreign interference or theft from an untouchable overseas company, which doesn't seem to be the case here)