So the fact that someone lied to you to get you to kill a member of your own family doesn't really seem to bother you all that much, huh?
Actually, it doesn't seem like she or he have acknowledged that she's his auntie. I was miffed at this too, but I think I have a reasonable explanation (though I'm only reading this now, so I may be way off by next chapter).
Simply put, she's a sore point for her sister and bright; not because they dislike her, but because they're said over her (believed) death. You probably don't (or more hopefully, wouldn't) talk much about dead friends and loved ones, and if you're particularly sore over it, you may never speak of them despite their importance to you. This can be a similar situation.
Add to this that the mother (the sister) had her give her life in her place, and the father (bright) asked her to do so - essentially making them feel like they murdered her.
So I can see why the MMC knows nothing of the 'witch' outside of rumours, despite how galling that is. I do hope the sisters reunite at some point and some apologising and chewing out happens though.