Children only.

Even though you might be related to a US citizen, that doesn't mean you are entitled to travel to the US as if you were a US citizen, especially once you're an adult...

Presumably there's some dual nationality or renounced American citizenship stuff going on here - otherwise the adult child of a US citizen is a bit hard to understand - as well as the sister presumably being in the US on a visa of some kind... so some further details would probably help.