That's an interesting one. Have you got to the bottom of why they are telling it to you repeatedly? I live with someone who very often asks me questions they know the answer to, and it sends me potty. If they have forgotten or weren't paying attention then fait enough, just tell me so and I will know where to start the conversation. But knowing it's all been talked about and conclusions reached, I get really anxious at it all being raked up with no explanation beforehand as to why. The last thing I need to hear is "are we still doing/going/etc..." (you can fill the rest of it), knowing full well if we weren't then we'd have all been notified.
I do consider myself lucky though, in the sense that this person's sister is even WORSE for doing it...I think it stems from their upbringing. I think also it is some kind of confirmation process they go through, but instead of simply saying that they put the onus on the other party and then project their own anxieties accordingly.
Furthermore, said sister of this person is one of those for whom there is never enough choice, and when presented with a plan of action will automatically prise the lid off it (so to speak), shake out the contents and sift through it, like a desperate parent going through the contents of a vacuum cleaner, seeking that one bit of Lego that's gone missing, before putting it all back together as it was.
Also, she wants to be invited to everything she can, but will back out at the last minute. She never, ever, wants to go, she just likes being invited and letting us do a shit-load of running around to accommodate her needs. Either that, or it's "yes I'd love to come, thank you. But not at that time. Or that day. And I don't want to go there". To this day she wonders why I keep her at arm's length.