To me, if voting for my uncle is permitted, then not voting at all is also permitted ... because it has the same effect, at the end of the day.
It depends. Usually when you go to vote, you have many, many races to vote on. If you voted for your Uncle in a race that had 2 mediocre/bad candidates, this is understandable. If you voted for your uncle when there was a legitimately good candidate, that's wrong. If you voted for your uncle in every race, I agree, you might as well have stayed at home.
But voting is more than just one race. Voting is much more than just governor or president or US congress. Judges, sheriffs, local city officials - these have NOTHING to do with abortion or national issues. But they do affect your local city and one has an obligation, both of charity and justice, to 1) help their neighbor by taking care of one's city, 2) to elect officials that will uphold and enforce (and pass good) laws.
The Constitution might give you the "freedom" to not participate in voting, but I think Catholic principles says you have an obligation to be responsible and a leader in local government (at least). Voting for a few hrs every 2 years is the bare minimum of "civic duty". If you don't take part in it, and you don't think anyone else has to take part in it, then a person has NO RIGHT to complain about ANY law that goes into effect.