] I also found these words of his to Elinor interesting: "Mrs. Ferrars does not know what she may be doing -- what she may drive her son to." I suppose he was remembering his own sad history again there, but I wonder what he meant by that?

While Edward's reaction to his mother's actions may not be the same as Brandon's to his father's, Brandon does know that when a man is desperate, he is capable of anything. He, not really knowing Edward, has no idea how his desperation may manifest itself, nor does Edward's mother.