Hey, thank you for bringing this up to me! I haven't thought much about the fact that her canon might be upsetting or triggering. I remember around when I first joined the game with her, and mentioned that she had a complicated canon and mentioned the problems with Kaname, as well as the incest. And the response I had overwhelmingly (granted) through plurk, was that almost everyone's canon's were complicated- especially CLAMP ones- with bad relationships and dramatic family affairs. And that it would be absolutely no problem at all.
Due to that I hadn't thought much about it since then, and that's not a problem I've run into in any of my other games over the years before. So I apologize if anything Yuuki has said to your character has been upsetting. That's not my intention. My permission post is definitely for all games. I linked it on plurk, once, but again- it might be better if I put it alongside my stats. People are welcome to tell me that they want to opt out of interacting with Yuuki or certain aspects of it. I definitely would like for individuals to be up front with me if something in my playing makes them uncomfortable.
In regards to Kaname- he's not here anymore, so as you said, it shouldn't prove much of a big deal. But I will say that neither myself or the mun would have ever had Kaname attack another character or do anything to them without OOC permission and plotting. And while Kaname can be deadly- he wouldn't murder someone for looking at her or being her friend. Yuuki is afraid that he would because she doesn't understand his intentions. And Kaname is the type to use people (much like Fumito, Jamie, and others), but Kaname himself wouldn't go out of his way to actually hurt someone unless they hurt Yuuki or proved a great risk of some sort. That's her character worrying more so than what reality would likely be for the person. He's a jerk, but he doesn't seriously murder, stalk, or kill for petty reasons. Or rather... the only person he's likely to keep an eye on would be her.
I don't... think Yuuki has told many people that Kaname is (was) her fiance and her brother recently. If she did and it's slipping my mind, I apologize. I know I often talk about it and her vampirisim on plurk when I link posts, so I usually assume that the muns are aware. For my permission post I'll ask that people please tell me up front whether they want vampirism or incest touched upon at all so that I can avoid it with that character altogether. And if something new suddenly happens, will try to make an ooc note. The permissions will help me as well, because if I feel that I need to walk on eggshells around certain characters, or restrain characterization heavily due to what she is, I'll look for her CR elsewhere.
Basically- I'm fine with all of this! And thank you for bringing it to my attention, since I had been under the impression it was not a problem before. I'm just going to ask upfront in my stats that people tell me directly what they want, because otherwise I will default assume that things are okay if someone posts to her, and should something be brought up the character/mun will be able to handle the information.