My sister’s coming over. You could at least throw on a slipcover.
People often clean up the house and put on better clothes when guests are coming over. Here --- the joke is that the woman sees her husband (who is a couch) as completely naked, which would be inappropriate for her sister to see. So --- instead of encouraging him to put on clothing (a more usual case, if he were just a normal naked person), because he is a couch, clothing for a couch would be a slipcover, i.e., a couch-shaped cloth covering that people sometimes install to maintain their furniture's quality.