There's obviously something extremely wrong when a half brother and sister have been in a sexual "relationship" (I use inverted commas since evidence suggests this is a cycle of abuse rather than a true loving relationship) since they were in their teens (if reports are accurate - if they're 35 and 39 now and SS were aware of the relationship in their 90s, they would have been early-late teens when it became obvious to other people).
The Guardian says that Barrass suffered sexual and physical abuse and was put into care herself. I would bet money Machin her half brother was involved in the sexual abuse and was almost certainly abused himself but now I'm speculating
I only hope the remaining four children can break the cycle their parents and grandparents have been perpetuating for god knows how long.