So I misunderstood the conversation that FSIL and Dark Sister had. Now that I understand it, it's worse.
Pretty much, FSIL was talking about how she didn't like how her students dressed because it portrayed their immaturity, that it was inappropriate, et cetera in front of Dark Sister who was pretty much wearing those exact types of clothes (out of four items, for example, she was wearing three). FSIL then, in trying to explain a particular outfit, looked at Dark Sister and said, "Well, you understand" in a condescending tone.
figee - I've been accused of that, too. Usually I'm just trying to explain something in great detail so others understand, but I've been told it comes across as if I think they're stupid. I really try to make sure I don't over-explain things anymore.