flower, what do YOU think? I would say it depends on what YOUR house rules are. If your children are having issues like these, it would be productive to have a family meeting, discuss these things, and agree on house rules. Then write them down and post them. At the time of the meeting you should also discuss what the consequences are for breaking the rules. In other words, if the child who breaks the rules gets a time out, goes to their room, loses a privilege, etc. You can also discuss things such as “if you see your sibling reading the book or magazine ask if they are done with it” or “if you are reading a book or magazine put a placeholder in so someone else would know that you are not done with it. If there is no place holder in it, then it is up for grabs”.
By discussing these issues with the kids and making house rules, you are discussing the importance of having “Respect” and consideration for one and other. Establishing this in young children at a very early age is important because you are establishing a foundation for respect and consideration for each aspect and relationship of their life.
For instance, if you are sitting on a chair in shul and you get up and walk away not leaving a siddur, sweater, hat or asking anyone sitting next to you to mind your seat because you are coming right back, do you have a right to tell or even ask someone to get up and give “your” seat back to you when or if you return? If you had already established rules in the home this would not be an issue. It would seem rather rude to ask someone to get off a “public” chair in shul unless it is a makom kavuah and everyone knows that is your chair. Basically everyone would have the same right to that chair. So having established the rules of respect and courtesy in home early in childhood, there would be no such issue with that child outside the home as they grow up.