The club has a system in place which prevents away fans (unless they know a Brighton fan) from buying tickets in the home end. If your family are Brighton fans (and I don't know you so I have no reason to think they aren't), and they have no purchase history, then it will be as difficult for...
If it was legal to sell tickets and BHA had no problem with it, I can see hefty amounts being paid for any seat for this game.
Thankfully though, that is not the case, unless people want to put their season ticket at risk.