Let's take a step back for a moment and think this through.
According to the OP, the uncle and niece slept together twice, once when she was 15 and the second time when she was 18.
At age 15 there are issues with the niece's inability to consent due to her age, along with the crime of incest. However, at age 18 consent is no longer the issue but incest is still a question. New York Penal law 255.25 defines incest as (among other things) being certain sex acts between uncle and niece whether their relationship be by blood or marriage. So in the second incident, the niece would be equally open to prosecution for incest as the uncle.
In pursuing such a matter you cannot complain about the the uncle having sex with the niece when she was 15, but have selective amnesia about the two of them consensually having sex when she was 18.
So, letís think this through for a minute. The family convinces the niece to report what happens. The niece says it happens, the uncle denies it. It's she said/he said and absent additional supporting evidence, the case is likely to go nowhere. However, the niece has confessed to voluntarily committing incest at age 18, a felony, even though her co-conspirator denies being a participant. In doing so, police reports (public documents) will have been prepared, memorializing her confession to felonious acts, damaging her reputation forever. All of this will have been done to satisfy the desire for revenge by third party relatives who were not a part of the event and who were never damaged by it in the first place. Remember the law of unintended consequences. Is this what the family really wants?
Don't wast money buying a plane ticket back to New York. Don't waste the police and prosecutor's time on a case against the uncle that will go nowhere. Don't put the niece at risk of prosecution by asking her to confess to a crime just to extract revenge against the uncle. Just settle this within the family.