Don't worry too much about it. Not all kids twiddle, and you can usually stop the habit from developing simply by covering the unused nipple. You can't twiddle something you cannot grab. Many babies will knead the breast as they nurse- it helps stimulate milk flow. But kneading is different from twiddling, and generally not as annoying. Just keep the little fingernails trimmed.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"