I'll put it this way: why is it ever OK to edit somebody else's PC background/history against their will?*
* besides the obvious need of the DM to ensure the PC fits the campaign world, e.g. "No, you can't play the child of a cybermage from Antares in my 10,000 BC low- magic, low-tech campaign!"
I might be contradicting myself a little here, but there is editing, and there is EDITING...
First off, I'd never allow another PC to do the editing. Not because they would necessarily do a poor job of it, but because it creates a fair-play field. A player's choices about his PC are HIS, and I don't think its any fairer to let another player make an character history edit (I'm his secret sibling or ex-lover) than I would allow another player to roll his HP when leveling up (or similar mechanical change).
If both players agree (We want to play brothers! or Our characters hook up. I've actually DONE both) then I allow. I NEVER would hide such info from my players though; they're mature enough to know how to play dumb on things like trolls/fire or Strahd's a vampire, they can handle a not-so-secret secret admirer.
HOWEVER, the PC backstory editiing rules don't necessarily apply to the DM, as long as its done in fairness and to further the "story". I usually seek the player's permission first (hey, are you willing to take a little tinkering for a really cool plot point?) and I do it very sporadically (It was cool when we learned Vader was Luke's father all along, it was less so when we learned Leia was his sister all along...) I've introduced a few such elements, but usually what I do is ask players for backgrounds and twist them a bit to make either cool adventures out of it or to fit better with my world. I'm also not afraid to back-track a bit (or ret-con) to fix problems if someone complains.
Lastly, you GOT to understand your audience. Lanefan's audience obviously doesn't care about things like secret admirers, party-assassins, or the like. I've had players (male and female) who objected to worshipping non-Christian gods (they either avoid clerics or worship non-specific "goodness" in the world) and others who were uncomfortable with the idea of a role-playing a relationship with someone that wasn't their BF. I've also had homophobic players who didn't like the idea of two dudes (one of which was playing a female) RPing a "relationship" and didn't want anything to do with it. In each case, we found work-arounds, or dropped the idea utterly on first objection.
In NO case was anyone's character built around the idea, or felt the need to pursue it beyond the first objection. I'm sure anyone trying to do so would get you booted so fast from the table you wouldn't have time to grab up all your dice! We gave up on using D&D for head-games/revenge/stealth dating in high school and DON'T miss that element of drama.