Should I lie to my sister about being a Christian?

The point of being a God parent is that you are willing to make a commitment to the child's spiritual future, you will be asked at the service to renounce the devil etc, so I suppose there is a requirement that you are a Christian. My advice would be to explain to your sister your feelings, stress that although you believe in God you are not drawn to any particular faith. In the end it might not be a bad idea to have a word with her priest, suggest that to your sister and say you would like her there to as you want to be part of her daughters future.