Its more often than not, stress related so it is possible that another move could trigger it again, but if hubby's aunt and uncle make the move as stress free as poss, with feliway plug-ins and a room set aside for the settling in period, it should lessen the possibility of another bout of cystitis. The other thing I wondered about is if he actually gets on ok with his brothers? (just because they are brothers doesnt necessarily mean they will always get on) and that could be a source of stress in itself, but may resolve once they are all in a proper home environment.
Of course he may never have another bout, but certainly a wet diet (high meat/low or no grain) is recommended and if it did turn out he is prone to it then there are products that can help such as Cystaid (which is glucosamine based).