I'm just trying to come up with ideas here so I'm just kinda spewing out what ever comes to mind. If you want them at the party it could happen there just has to be a reason to have them there. They could be there for any number of reasons such as curiosity or concern for what exactly is being planned. If they're concerned about their family then it's possible they could consider this 'party' a threat, so just to be on the safe side, Itsuki could attend.