"representing a borough in three successive parliaments, exertions of loyalty, and dignity of baronet, in the first year of Charles II."

Sir Walter Elliot is a baronet.  In the English nobility, a baronet is usually higher in status than a knight of the realm, but lower than a viscount or a baron.  Lady Russell is the widow of a knight, so, in order of precedence, she is lower than Sir Walter who is a baronet in his own right.

Sir Walter's title is not sufficiently powerful to enable him to pass on courtesy titles to his female children (notice the Viscountess Dalrymple's daughter has the title 'Honourable' in front of her name and Elizabeth is 'Miss' Elliot).