I mean it's perfectly legal and no-one will care how many adults are living there because at 16 and 19 people are grown up so it's none of anyone else's business how many people are there within reason.
slight correction to my post, as long as the 2 females are sleeping in one room and the male in another it is quite legal and not even counted as overcrowded. The room for sleeping does not have to be a bedroom it can be any room ,( kitchen ,lounge) but not a bathroom or toilet.
However this would give you extra points on the local council housing list if a non bedroom has to be used for sleeping.
Has anybody read mamya's Shelter link because it seems to me that they are overcrowded as the law stands at the moment. But I've got a headache and might not have read it right. :-(
If its council property then you can complain and argue that its not a big enough property for you. But if it is your own property - then surely its no one's business. Is there a legal side to this? I dont think so.
However, you must be a bit overcrowded. If I was a 19 year old girl, I certainly wouldn't want to sleep in the same room as my brother and parents.