Unless your nephew is somehow listed as one of your travel eligibles (i.e. 2nd enrolled friend?) I would think the UM fee would apply. Unless you are saying he would not be a UM because you are accompanying him, in which case it seems like it would work so long as there are seats on the flight he is ticketed on. I do know that if you end up in the jumpseat, most gate agents will want to make him a UM since you are not in the cabin, but that probably depends on the agent, the FA, the kid's age etc. Just guessing, I've only done it with my own kids.