That Scarlett was loving Ashley, that her marrying Charles was to spite Ashley? The book mentioned, Scarlett wanted to tell Ellen everything when she saw Ashley was with Melanie, but decided not to do so later on. Gerald knew everything but he was too careless to mention it to Ellen.
I don't think Ellen knew. She did however, know that Scarlett was being fast after Charles' death by flirting around and wearing colors instead of the usual black clothing of a widow too soon. Yes, Scarlett thought of telling her, but she never actually got to.
Ellen had had her own indiscretions in the past with her first beaux. I don't remember his name. Phillipe? I think she new what Scarlett was capable of but chose not to believe it.
I think Ellen was very well aware of Scarlett's true nature. A mother doesn't miss much. Ellen was a naturally gentle and patient person and so she never dealt with Scarlett's misbehavior in a harsh way, but I'm quite sure she always knew exactly what her daughter was up to.
What I got from Ellen is that she wants to see the best in people. So while she knows that Scarlett is not perfect and tries to mold her into what a respectable young Southern lady should be, she doesn't think Scarlett would go so far as to marry a man she doesn't love to get back at an engaged man.
The book mentioned, Scarlett wanted to tell Ellen everything when she saw Ashley was with Melanie, but decided not to do so later on.
Gerald knew everything but he was too careless to mention it to Ellen.