can I sue?

For private companies there are no laws against nepotism.

No, they will do that, look for another job or put up with it.

You can sue anybody for anything and like all losers, you'll lose.

Doubtful. There are very few laws that address nepotism and those that do typically only in regard to public-sector employment. And get use to some people being favored over others.

If you can prove monetary damages, yes, you can.  I suspect that when your mental health is examined, you are going to lose:  "I am making deliveries for a living and have been doing so for 30 years now in California. I would rather be dead than to have to drive anymore."

If you work for the government, and can prove your damages, yes.  But if you work in the private sector, no.

As always the answer is ... YES ... you can sue.  The real question is, can you win?  Is there a nepotism tort in your jurisdiction?  Can you prove damages?

of course not. That is not illegal.