On the part where Kevin is taken up to the attic for bed, his mom says that there are 14 people at the house. When Kevin's sister counts everybody when they are leaving for France, she tells her mom that there are 15 people altogether.
His mother was probably referring to everybody except Kevin . When Kevin's sister was counting she was counting Kevin too ( although she counted the wrong " Kevin " ) .
When Kevin's mother is pointing him towards the attic stairs, she says, "There are 14 other people in this house, and you're the only one that manages to get into trouble!!" I don't know if she uses those exact words, but she does say 14 other people!