You assume rescheduling is easy and cost free in each and every instance. If their tickets and arrangements can be rescheduled without cost or the family reimburses, then no harm/no foul. But if people other than couple and mourning family stand to lose money without reimbursement, then yeah. They are kind of obliged to not waste the money of people who have reasonably relied on invite. As for monsters, it is one thing to have a tradition. But they are lashing out and not handling this dispute politely. I wonder if they really like OP to begin with. If they dislike OP, they may have justification. It sounds like OP and fiance might be mismatched.