I'm going to make my sister pick up my nephew early
Will that affect your sister's schedule in a negative way?
No, but I do think that there has been tension between us for a while.
Maybe ask her if she could pick her nephew up early? you don't want to upset her and make demands. It sounds like the two of you should discuss what's causing the tension between you and try to work it out.
It's my nephew, I can make her do anything I want.
Try to talk to her and work on the issues between you. I think approaching the situation with understanding and kindness will improve your relationship going forward.