Plot altering powers

[Blasted]

Recently I've been addictedplaying through fallen london, a reasonably simple browser based time waster, with decent writing. You get dealt a number of 'opportunity' cards, some of which begin or alter plots.
In a recent thread there was mention of plot altering using xp, effectively exchanging it for permanent power gain. Which seems foolish in most circumstances, so in time honoured RPG gamer manner, I'm considering a heartbreaking concept.

Something along the lines of "Choose 2 of six cards each level, to play during the game"
Each of the cards would have something along the lines of "It's your long lost sister!" or "The map to Xanadu!" which could be played during an encounter or a treasure haul respectively. This would in turn alter the plot, introducing elements as determined by the players.

It has several obvious drawbacks: The additional work for the MC in weaving in the dispirit elements. But also the problem of players overriding each other's plots ("She was your sister, but now is my uncle.") I imagine that balancing the cards would be a further issue. Finally, it relies on that most rare of beasts: the decent MC, who will refrain from staining your cheerios.

Obviously to be an addition to an existing system, are there any attempts at this which by-and-large succeed? Fail less than dismally?