Perhaps there is recorded documentation in this country of her actual age, she came here with her father as a child and he must have filled out immigration paperwork for himself and his children and age would almost certainly be part of that
If she changed her birth year to make her appear to be 17 instead of 18 it would show she was never naturalized and made a US Citizen, because she was an adult when her father was naturalized and not a 17-year old child.
Besides committing fraud by marrying her brother to allow him into the country,
she committed fraud by changing her birth year to help her gain citizenship as well.