it could be the spirit of a kid that's always been in the house.
it might not have been threatened when the couple was just that, but when a kid came home it cracked the shits.
similarly, when the sister / husband / kid moved in for the week, the kid may have threatened the spirit.
also, has the kid been present in every room when the incidents
happened?
has the kid reacted at all, or changed demeanour during the incidents?