Bets that the heir is going to marry one of the younger sisters?

It could all have been avoided, too

No, no. That would require research, not reading "How We Lived Then" and being married to Mrs Fellowes (though if Cora's father had been alive and the fortune been his, would this still have applied if he hadn't actually given it to her - as opposed to his not realising that it would happen by default if he did?).