"the only true way to protect her." I'm not sure that seems very realistic!
What seems to be missing here is that foster parents are put through a process to assess them and they do seem very dysfunctional - how did they get through that proccess!
I don't think it can be the foster mum or dad, as both believe it may be their son!
Strangling is such a specific crime, well to me, it seems to be. Not something that could be done by accident.
In Agatha Christies' it is always the least likely person, at the moment the least likely person is the Granddad, so my money is on him! But actually I don't think he did it either!