My wife does upvote my posts, and I upvote hers... that's a slightly less insidious form of nepotism...
If it's "social media", then I would expect that family and friends would get more up-votes than random strangers. That's part of the equation when you're evaluating subjective value from your own perspective.
I don't even have a problem with self-upvoting, provided that it's not coupled with spammy content. IMO, the problem isn't self-voting. The problem is the incentive that self-voting might create for people to intentionally produce and vote on low-value content. That would be sort of like a form of embezzling.
We paid/contributed to earn the right to vote. That includes the right to vote on our own stuff, and our family's.
You're exactly right, the problem is the lack of incentive to vote on others.