Why is Lockwood confused at first about the social positions of Hareton Earnshaw and Catherine?Wuthering Heights by Emily Bronte

After his rather odd first reception at Wuthering Heights,
Mr. Lockwood who perceives Heathcliff as a misanthrope like himself, returns for a
second visit from Thrushcross Grange having decided to leave his residence because a
servantstirs up ashes and clutters the room.  However, once he arrives at Wuthering
Heights, he again encounters antisocial conduct from its
inhabitants.

When no one answers the door, Lockwood
encouters the "vinegar-faced Joseph" and inquires why no one has responded.  Joseph
curtly tells him that the young miss will not answer the door.  At last, a young man
with a pitchfork appears and opens the door for Lockwood where he he observes the
'missis' whom Joseph has mentioned.  Although she has "a most exquisite little face,"
there is a look of desperation upon it.  Rudely, she refuses Lockwood any tea until
Heathcliff enters and "savagely" bids her pour the man some tea.  Then, in an effort to
be amiable, Mr. Lockwood says,

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it is strange how custom can mould our tastes
and ideas: many could not imagine the existence of happiness in a life of such complete
exile from the world as you spend, Mr. Heathcliff; yet, I'll venture to say, that,
surrounded by your family, and with your amiable lady as the presiding genius over your
home and heart—”

To these
remarks, Heathcliff responds "almost diabolically," asking who is his "amiable lady." 
Mr. Lockwood has confused Catherine Linton as Mrs. Heathcliff.  But, upon closer
examination, he realizes his error as Catherine is very young, and Heathcliff around
forty,

a
period of mental vigour at which men seldom cherish the delusion of being married for
love, by girls:  that dream is reserved for the solace of our declining
years.

Then, he thinks that
Hareton, the young man may be the girl's husband when, in reality, they are cousins. 
Without explanation, Heathcliff identifies her as his daughter-in-law, but she was
married to his sickly son Linton by Isabelle Linton, who have both died.  Hareton is not
identified by Heathcliff, but he is the son of Hindley and Frances Earnshaw, who are
both deceased.  At Wuthering Heights, Hareton is treated by Heathcliff much as he was
treated as a boy himself.