If you're escorting a minor child to the gate, are you allowed beyond security without a boarding pass?

Yes in fact if she is flying as a (UM) you have to go to the gate with her. When you check her in you get a "gate pass" this will allow you to the gate with her. You must have ID. You don't have to pretend to be her guardian. If you are the one taking her to the airport you can get down there. Also if she is flying as an UM you must wait at the gate until the plane leaves. You will have to fill out paper work saying who will be meeting her on the other end. And they must have ID and it must be that person taking her. (IE if you have Aunt Sue as the one to pick her up it Must be Aunt Sue. Uncle BOB cant pick her up.

Go to the airline ticket counter and explain that you are the guardian of the child until they reach the gate. The airline will give you a special pass that gets you past security, although you must still obey all security rules just as if you were a passenger. You will then need to get to the ticket agent at the gate and transfer responsibility of the child to the airline representative (a.k. chaperone). On the other side, the child will only be released to an authorized adult that has been chosen in advance.

when your sister gets her boarding pass you can get a pass that will allow you to get through the security gates. You should call the airline to be sure of what identification you will need.