Traditionally, the godparent(s) witness the christening and commit to help the child learn their catechesis and help them in their spiritual formation. In modern times though, it could just mean a commitment to be the child's guardian if something happens to the parents. I'd ask your sister to clarify what she expects from you as a godparent if that's the only thing keeping you from saying yes.
I'm not anti-Christian. I'm anti-stupid.