"on her [Lady Russell] kindness and advice Lady Elliot mainly relied for the best help and maintenance of the good principles and instruction which she had been anxiously giving her daughters." (ch. 1)

It sounds as if Lady Elliot knew for some time that she was dying and had kind of bequeathed the education of her daughters into Lady Russell's care, for she knew Sir Walter was "a conceited, silly father."

Elizabeth was 16 when her mother died - old enough to have absorbed the "good principles and instruction" given by her mother - and she had two more years with her than Anne had, who, of course, absorbed plenty.

Ch. 1 also says that "soon after Lady Elliot's death…when Elizabeth was in her first bloom" - so 17, 18 yrs old? - she went to London and the whole Mr. Elliot thing happened. Elizabeth's "strong family pride could see only in him, a proper match for Sir Walter Elliot's eldest daughter. There was not a baronet from A to Z whom her feelings could have so willingly acknowledged as an equal." So, very shortly after her mother's death, she is quite full of the Elliot pride and snobbishness.