I read this back in 2008 or so, but I have no idea how much older it is. I remember at first you follow this person who plays a game, and along with a lot of other people, he gets sucked into the game, and I remember that one of the characters is the little sister to one of the others, although I don’t remember if t was his sister. They meet up with the others in the game, and there’s a whole thing with them realizing they can’t harm each other, and then trying to kill some in-game monkeys. At some point, they get attacked or something, and one of the characters disappears when his health ran out, and the others worry if he died in real life as well(it was revealed in the end that all that died in the game survived I think). I remember them towards the end of the game meeting the creator in the game, who mentions something about using the software to try on clothing from online stores or the like. One of the last things I remember of the plot is them nearing a fort or castle or something with enemy guards, and I don’t remember how exactly but they were about to lose or something, but the sister I mentioned earlier(who if I remember correctly was just there and don’t do much until now) did something that saved them. I seem to remember that the cover was Ana illustration of the kids in what looked a battle situation and one of the kids pointing in the direction they have to go.

Any help remembering this is appreciated!