Can my wife and I be godparents to a different child if we are already godparents to our nephew?
If you are Catholic you would know that
Yes. And if you were confirmed, practicing Catholics (which is required of at least one of the godparents, otherwise you're a Christian Witness), you would already know that.
I don't see why you can't be godparents to both children. I think the only rules that really apply to these situations are the ones the parents and selected godparents choose to abide by. Although it seems a little weird for the family friend to ask you to be godparents when you're not very close.