I think a simple and elegant solution to the cousins thing is to make Bruce a 2nd cousin to both Bette and Kate, like this.
Roderick had a brother who fathered Jacob, Phil and unnamed. But the brother died when they were infants or young
and he adopted and raised those boys with Nathan and Martha. They're 'siblings' due to adoption
but they're also first cousins.
Children of first cousins, Marta > Bruce, Jacob > Kate, are 2nd cousins.