I think the actual person who would have to move trumps the potential people who might have to choose different seats but wouldn't have to move since they weren't there to begin with. She found a seat that hadn't been claimed yet and sat in it. Then your family showed up and tried to run her off just in case you needed her seat for someone else. How was she supposed to know that the entire table was supposed to be saved? A couple of people had apparently claimed seats at the table. That didn't mean the whole table was reserved. Even
you didn't know if you would have enough people to fill the entire table, so how was a complete stranger supposed to know that all the seats were saved for people who potentially
might show up?
I go to them often - including a banquet last Friday and a concert on Saturday, and I have to go to more this week. At the banquet, each table held 8 people.
Two backpacks at one of those tables would "reserve" two seats at the table, not the whole table. Two backpacks on a row of 20 seats in an auditorium would reserve two seats, not the whole row. If the OP's family wanted to be absolutely certain to have those specific seats, they needed to get there early enough to be sitting in them before someone else decided to sit there. Another option would have been to have someone physically there to tell everyone that the seats were being held. Otherwise, they should have placed an item on each seat they wanted to hold, and hopefully no one would have moved the items to take the seats. Claiming a few seats at a table or on a row isn't going to communicate to everyone else that the entire table or row is saved. Can you imagine if everyone assumed that a table or row was reserved just because one seat at the table or in the row was saved? If that was the case, the vast majority of tables at our banquet would have had only two or three people at them, and everyone else would have been standing around until the banquet started!

Certainly when someone is sitting at the table or in a row, it's polite to ask them if the other seats around them are open or they are being saved for someone else. But when no one is there, it's completely reasonable to assume that the only seats that have been claimed are the ones that have items on them and that any empty seats are up for grabs.