You have a family of six, you should buy (at least) a six-seater.
@Vilters' post about fuel/load etc. is very valuable, but you have some options.
I'm guessing by your age that some of the gang are small/very small, it may also be that you're not planning on flying all of them long distances.
Our FG Saratoga can take 6 adults (no fatties and no luggage) 400nm. It's old, as they get newer they carry less. Or, I can take four adults and luggage almost 1000nm...
Cheers, Sam.