If she's looking for the response, it will be hard to get your 5 year old not to respond (which would soon make the "game" no fun). So instead immediately remove the younger daughter from the room - say to a playpen in another room for a couple of minutes. She will learn that the response to pinching is her leaving the room.
But another piece of advise - watch and listen to what is happening just before she pinches. She may be doing it just to watch her sister scream, but often toddlers without a voice use pinching, biting, hitting to communicate their frustrations. It is possible that the 5 year old just snatched a toy or refused to share a snack or whatever else. When my then 2 year old went through a biting phase, it was almost always because the older sibling was ignoring her or annoying her. If you will teach your 5 year old to listen to the toddler (which at this point may just be her pointing at a toy she wants) and cooperate with her when possible, it will reduce the toddlers frustration - and her need to communicate in ways that obviously get her point across (ouch!).