Huh. Why are ascribing motives to any of the players?
Huh, indeed.
How is it any different that I can play a character that I KNOW makes another player uncomfortable to the point where that other player might actually leave the game, but I can't play a character that takes away someone's imaginary friend's imaginary sword?
Huh?
But, it's not okay for me to play a character that takes away someone's magic sword?
Why not?
Huh?
I'm afraid you are just confused, and little is going to be gained by attempting to correct you.
My standard with regards to backgrounds is pretty simple, and I've outlined it several times before. If it's not clear now, I have little hope it will ever be clear to you. But, once again:
I won't let you create a background that implies or requires explicitly or implicitly any interaction, association, or action on the part of another players character. You don't get to determine how some one else's character behaved in the past any more than you get to determine how another character will behave in the future. Thus, you don't get to create a background that involves a relationship with another PC - whether sibling, lover, or simply just friend - without that PC's permission. Players are encouraged to work together to come up with ties between their characters, but I won't approve a background like that without the consent of both parties regardless of the motives and intentions of the players.
Now, if you wanted to create a background that involved a purely unidirectional relationship - you don't know the character but you've always admired some other character at a distance - then that might not be the wisest idea but it is at least in your rights as a player. Indeed, such a one directional relationship is not something I as a DM have much control over, because it can just as easily be an in play artifact as it could be a background and as the DM I can't tell anyone how to play there character because that would be overstepping my authority.
A 'secret' sibling that you never knew about skirts the line here. Technically the relationship is unidirectional, but blood ties like that are so personal that I'd probably nix it without consent of all parties. On the other hand, I might create a secret background that neither player knew about that made them both related if I think their backgrounds warranted it and it would make for an interesting story. If you can immagine the Star Wars characters as a PC party, the Darth Vader character and the relationship between Luke and Leia is a secret background of this sort. Hense, the reason that if you really don't want me to 'mess' with you, create a family - don't create think that you are going to get away from story complications by being an orphan.
The rest of my standards on character background aren't really relevant to the thread, but just for the record.
a) You have to play a character who is willing to have adventures.
b) You have to play a character who can get along with the rest of the party at least sufficiently well that the party doesn't split.
c) Your character has to work in the setting of the campaign.
d) Your character can't draw any significant benefit from his background without spending a trait to gain the appropriate advantage. You can be the King's son, but you have to either pay for it or expect to gain no benefit from it (you've been disowned and you are an outcast in a foreign land, whatever).
As far as rewriting someone's background goes, well, if someone has written that their family was killed, that's a pretty strong signal that they're not interested in playing out family relations in the game. Now, let's say that's the background. Bob's character has no family because they were eaten by vampiric wombats.
You introduce an NPC family member.
If you write a background in which your family is killed by vampiric wombats, it almost 100% certain that some point I'll introduce a vampiric wombat to the game that just happens to be one of your dead relatives. If you write that background, I'll see it as practically begging for that plot line. It's like hanging a rifle on the wall in scene 1. You ought to know that its going to be fired by scene 3.
Bob, after the session, because he's a good player and doesn't want to derail things, says, "Dude, I'm so not into this. I don't want to do this. This whole plot line you've introduced that contradicts my character background does not interest me in the slightest. Can we stop?"
What is your response?
a) "Thanks Bob for waiting after the session to discuss this. Now we can talk about this at length without boring everyone else."
b) "First of all, I haven't done anything that contridicts your background. I'm simply filling in blanks in the story based on cues in your player background. I'm doing this to help you develop your character, and not for any other reason."
c) "Secondly, if you are really not interested in it, I'll drop the story line quickly and we can move on to something else. I'm sorry this plot development doesn't interest you. I thought based on what we'd talked about before the campaign regarding your goals and character conception and based on how you filled out the campaign questionaire that you'd really be into this, but I guess I was wrong. Maybe we should talk some more about what sort of stories you are interested in."