Paul's Case: A Study in Temperament
We don't even know how many sisters there are, but we know that Paul finds them boring and stupid, just as he does everyone else on Cordelia Street. (To be fair, their conversation about "how many waffles some one had eaten at the last church supper" does sound a little boring. It also makes us want to buy a waffle iron.)
Mostly, the sisters' presence in the story highlights how little (if at all) Paul feels connected to his family.