Yes, she knew. The scene with her and blondie at the beginning of this episode illustrated that they all grew up together. Elliott and his sister were childhood friends with her, and still are. Elliott is the one who has a hard time remembering things and deciphering fantasy from reality.
Slater (Mr. Robot) throwing Elliott off the railing still doesn't make perfect sense to me, because they still haven't given a clear answer whether Mr. Robot is a hallucination of his father, someone that looks like his father, or the far-fetched idea that it IS his father and is somehow still alive despite claims that he died. So was he pushed by an actual person, or did he jump? I'm still uncertain.
I've read 4 different recaps of last night's episode and every one has a little different perspective. This show keeps me guessing and keeps me on my toes. It's great.