I recall reading that Susan had spoken to someone from her church about marital problems, and that there were money issues, etc. I guess the husband could use that to bolster his argument that she left him for another man, but most women don't leave their kids.
I don't know what the situation with the FIL was, if he lived near them at the time. Was he even around when she disappeared? I'd have to know that before I suspected him.
In any case, I hope this one gets solved and the guilty parties get what's coming to them.