You have to verify your relationship with your uncle since he’s going to sponsor you. About his children, consular usually doesn’t go in detail about family members of a sponsor. During period of about 15 minutes of visa interview, he tries to verify whether you are a genuine student, whether you have ability academically and financially to study in USA, and if you’ll return back after studies. Students are generally not asked about family members of a sponsor. However, don’t lie if you’re asked but don’t give information regarding this issue until and unless you’re asked. When you tell him the truth, rest depends on the mood of a consular. He may not take it seriously because when your uncle is ready to sponsor you he has nothing to object.