Since John Dashwood's opinion of people is directly associated to the property they own he may thinks everyone including Miss Morton has the same standards. That's why I understood the part quoted from chapter 41 as John Dashwood's view of Miss Morton's opinion, but not necessarily really her stance. Even if Miss Morton did know and like or even want to marry Edward and/or Robert, John probably assumes that she wants to wed the heir of the Ferrar fortune and not a specific individual, so if Edward is the heir she will want him, if Robert will inherit then it's Robert she needs and "there can be no difference; for Robert will now to all intents and purposes be considered as the eldest son".