Accommodation Wording

I would say something like "Limited accommodation at the venue is available for our guests on a first-come-first-served basis. Please let us know if you would like to reserve a room."

I think that most people will realise family & wedding party take priority. There's no need to spell out that you have already earmarked most of the rooms for family. If all the spare rooms end up allocated, just tell anyone else who inquires that they either need to look elsewhere or that you can add them to a waiting list in case someone has to drop out.