1
She is my eldest sister.
To sign this, start by pointing to the person for 'she,' then sign 'my.' Next, perform the sign for 'eldest' by pulling down from your chin and moving the fist up. Finally, sign 'sister' by bringing your dominant 'L' hand down to your non-dominant 'L' hand.
2
Who is the eldest child?
Begin by pointing or setting up the group of children. Sign 'eldest' with the smooth downward-then-upward motion, followed by the sign for 'child' (patting an imaginary head). Finish with the sign for 'who' while furrowing your eyebrows to clearly indicate an open-ended question.
3
My eldest brother lives in New York.
Start with 'my,' then sign 'eldest' and 'brother.' For 'brother,' bring your dominant 'L' hand from your forehead down to your non-dominant 'L' hand. Next, sign 'live' by bringing both 'A' hands upward on your chest, and finish by fingerspelling 'N-Y' for New York.