My sister has gotten a ticket for coming up behind someone and flashing her lights...

While in general, I do pass on the left - and that's probably because I was taught to drive by my father who learned to drive in europe.... There are no actual laws that say you can't pass on your right in most states and most people weren't taught to drive by foreign parents, so expecting someone to get out of your way when it's clear to your right because you blinked your lights at them could be misunderstood.

I generally will just come up behind someone and keep a somewhat closer distance than they probably want, and hopefully they will then give way... if they don't, I'll pass on the right if it's clear.

Someone can look up the state laws if they want - I have no idea which percentage have pass on your left laws vs. not - but haven't had such in the states I've lived in.