My daughter started the same thing at about the same age and still does it occasionally. At first I went along with it and obeyed her "orders". I actually stopped giving into it one day because we were at preschool and she was trying to push another mom out of a chair, and the mom looked at her and very politely said, "You want me to move, but I'm sitting here right now. You can sit here (in another chair) if you like." DD was upset but I thought it was awesome that the other mom was very polite with her but at the same time let her know that ordering everyone where to sit isn't really acceptable behavior.

I started doing the same thing, and it works fine. She still does it occasionally, but it's definitely a lot less now. Although I think it's because she's grown out of it, not because of anything I did. Still, I think it's fine to let them know that everyone isn't going to cater to their whims in that regard, as long as you're gentle and polite about it.
However, if it really IS her chair (like one of her toddler chairs that goes with her table), then I always move. She has every right to order me not to sit in a chair that really does belong to her. When she's ordering me not to sit on MY favorite pappazan chair--sorry, tough patooties.

I'm allowed to sit down too. Heck, I don't get to sit that often--I need to take it when I can!
ETA: Yes, I'm pretty sure this IS an important step toward learning to share, and toward empathy. Incidentally, my DD is GREAT at taking turns now (most of the time, anyway)--which is the next step toward sharing, I guess.
