@maria1985,
And maybe it was not your sister's doing. Her son could have changed his mind last minute. Teens are like that. And considering he is a boy, they tend not to care (or act like that). It is likely he was months and months ago thinking that is an eternity before I graduate, I can't think of that now; who cares. Then all of a sudden it is before him. He sees his friends all planning parties and suddenly it is a big deal.
What Mame said, accommodate him if you can and if you can't just let your sister know, sorry XYZ cannot accommodate additional people. Look at it this way if you can accommodate, you are likely to get help now from your sister financially and planning wise.