Elizabeth1, Your brother in law cannot be discharged from hospital until a full discharge assessment has taken place. This has to be done by Social Services. Just make sure you are not included in the report as providing any form of support. As * JessM* says resist all emotional blackmail to include you in the care package. Sound, to them as cold and indifferent as you can, when they ask you to help.
I was pressured very hard by a psychiatrist to take my uncle home with me on discharge. I lived 'out of area' and it meant they could wash their hands of him, no follow-up, no care, no assessment. He played (or tried to play) on my affection for my uncle, shown by the support I had given him in the crisis leading up to and during his hospital stay. He threatened to just load him into an ambulance and dump him outside his home. But I just said NO. In the end he went into a Care Home for convalescence and as he had enough money could afford to decide to stay, close to friends in the town he had lived in for 35 years.
However, once he is discharged home with a Care package monitor it, and do not hesitate to try to get your BiL into a Care home if you think he is in danger in his home on his own.